Auto merge of #28663 - saschanaz:void-undefined, r=jdm

Convert Web IDL void to undefined

<!-- Please describe your changes on the following line: -->

Thanks to my tool https://github.com/saschanaz/gecko-webidl 🙌

Build is failing on my current VS2022+Python 3.10 environment, but the IDL tests are passing anyway, so...

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #27660

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- [x] There are tests for these changes

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159 files changed, 1162 insertions, 1144 deletions

diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index e8e5bfbab4f..046f7984858 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -1188,7 +1188,7 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
 
         return handleOptional(template, declType, handleDefault(empty))
 
-    if type.isVoid():
+    if type.isUndefined():
         # This one only happens for return values, and its easy: Just
         # ignore the jsval.
         return JSToNativeConversionInfo("", None, None)
@@ -1439,7 +1439,7 @@ def getConversionConfigForType(type, isEnforceRange, isClamp, treatNullAs):
 
 # Returns a CGThing containing the type of the return value.
 def getRetvalDeclarationForType(returnType, descriptorProvider):
-    if returnType is None or returnType.isVoid():
+    if returnType is None or returnType.isUndefined():
         # Nothing to declare
         return CGGeneric("()")
     if returnType.isPrimitive() and returnType.tag() in builtinNames:
@@ -4338,7 +4338,7 @@ class CGMemberJITInfo(CGThing):
                 result += self.defineJitInfo(setterinfo, setter, "Setter",
                                              False, False, "AliasEverything",
                                              False, False, "0",
-                                             [BuiltinTypes[IDLBuiltinType.Types.void]],
+                                             [BuiltinTypes[IDLBuiltinType.Types.undefined]],
                                              None)
             return result
         if self.member.isMethod():
@@ -4426,7 +4426,7 @@ class CGMemberJITInfo(CGThing):
         if t.nullable():
             # Sometimes it might return null, sometimes not
             return "JSVAL_TYPE_UNKNOWN"
-        if t.isVoid():
+        if t.isUndefined():
             # No return, every time
             return "JSVAL_TYPE_UNDEFINED"
         if t.isSequence():
@@ -4500,7 +4500,7 @@ class CGMemberJITInfo(CGThing):
 
     @staticmethod
     def getJSArgType(t):
-        assert not t.isVoid()
+        assert not t.isUndefined()
         if t.nullable():
             # Sometimes it might return null, sometimes not
             return "JSJitInfo_ArgType::Null as i32 | %s" % CGMemberJITInfo.getJSArgType(t.inner)
@@ -7336,7 +7336,7 @@ class CGNativeMember(ClassMethod):
                              # Mark our getters, which are attrs that
                              # have a non-void return type, as const.
                              const=(not member.isStatic() and member.isAttr()
-                                    and not signature[0].isVoid()),
+                                    and not signature[0].isUndefined()),
                              breakAfterSelf=breakAfterSelf,
                              unsafe=unsafe,
                              visibility=visibility)
@@ -7608,7 +7608,7 @@ class CallbackMember(CGNativeMember):
         convertType = instantiateJSToNativeConversionTemplate(
             template, replacements, declType, "rvalDecl")
 
-        if self.retvalType is None or self.retvalType.isVoid():
+        if self.retvalType is None or self.retvalType.isUndefined():
             retval = "()"
         elif self.retvalType.isAny():
             retval = "rvalDecl.get()"
diff --git a/components/script/dom/bindings/codegen/parser/WebIDL.py b/components/script/dom/bindings/codegen/parser/WebIDL.py
index d74278c3e0c..e317087837d 100644
--- a/components/script/dom/bindings/codegen/parser/WebIDL.py
+++ b/components/script/dom/bindings/codegen/parser/WebIDL.py
@@ -2093,7 +2093,7 @@ class IDLType(IDLObject):
         'utf8string',
         'jsstring',
         'object',
-        'void',
+        'undefined',
         # Funny stuff
         'interface',
         'dictionary',
@@ -2168,8 +2168,8 @@ class IDLType(IDLObject):
     def isJSString(self):
         return False
 
-    def isVoid(self):
-        return self.name == "Void"
+    def isUndefined(self):
+        return self.name == "Undefined"
 
     def isSequence(self):
         return False
@@ -2355,7 +2355,7 @@ class IDLParametrizedType(IDLType):
 
 class IDLNullableType(IDLParametrizedType):
     def __init__(self, location, innerType):
-        assert not innerType.isVoid()
+        assert not innerType.isUndefined()
         assert not innerType == BuiltinTypes[IDLBuiltinType.Types.any]
 
         IDLParametrizedType.__init__(self, location, None, innerType)
@@ -2414,7 +2414,7 @@ class IDLNullableType(IDLParametrizedType):
     def isInteger(self):
         return self.inner.isInteger()
 
-    def isVoid(self):
+    def isUndefined(self):
         return False
 
     def isSequence(self):
@@ -2517,7 +2517,7 @@ class IDLNullableType(IDLParametrizedType):
 
 class IDLSequenceType(IDLParametrizedType):
     def __init__(self, location, parameterType):
-        assert not parameterType.isVoid()
+        assert not parameterType.isUndefined()
 
         IDLParametrizedType.__init__(self, location, parameterType.name, parameterType)
         # Need to set self.name up front if our inner type is already complete,
@@ -2561,7 +2561,7 @@ class IDLSequenceType(IDLParametrizedType):
     def isJSString(self):
         return False
 
-    def isVoid(self):
+    def isUndefined(self):
         return False
 
     def isSequence(self):
@@ -2602,7 +2602,7 @@ class IDLRecordType(IDLParametrizedType):
     def __init__(self, location, keyType, valueType):
         assert keyType.isString()
         assert keyType.isComplete()
-        assert not valueType.isVoid()
+        assert not valueType.isUndefined()
 
         IDLParametrizedType.__init__(self, location, valueType.name, valueType)
         self.keyType = keyType
@@ -2673,7 +2673,7 @@ class IDLUnionType(IDLType):
     def prettyName(self):
         return "(" + " or ".join(m.prettyName() for m in self.memberTypes) + ")"
 
-    def isVoid(self):
+    def isUndefined(self):
         return False
 
     def isUnion(self):
@@ -2836,8 +2836,8 @@ class IDLTypedefType(IDLType):
     def isJSString(self):
         return self.inner.isJSString()
 
-    def isVoid(self):
-        return self.inner.isVoid()
+    def isUndefined(self):
+        return self.inner.isUndefined()
 
     def isJSONType(self):
         return self.inner.isJSONType()
@@ -2972,7 +2972,7 @@ class IDLWrapperType(IDLType):
     def isJSString(self):
         return False
 
-    def isVoid(self):
+    def isUndefined(self):
         return False
 
     def isSequence(self):
@@ -3178,7 +3178,7 @@ class IDLBuiltinType(IDLType):
         'utf8string',
         'jsstring',
         'object',
-        'void',
+        'undefined',
         # Funny stuff
         'ArrayBuffer',
         'ArrayBufferView',
@@ -3215,7 +3215,7 @@ class IDLBuiltinType(IDLType):
         Types.utf8string: IDLType.Tags.utf8string,
         Types.jsstring: IDLType.Tags.jsstring,
         Types.object: IDLType.Tags.object,
-        Types.void: IDLType.Tags.void,
+        Types.undefined: IDLType.Tags.undefined,
         Types.ArrayBuffer: IDLType.Tags.interface,
         Types.ArrayBufferView: IDLType.Tags.interface,
         Types.Int8Array: IDLType.Tags.interface,
@@ -3251,7 +3251,7 @@ class IDLBuiltinType(IDLType):
         Types.utf8string: "USVString", # That's what it is in spec terms
         Types.jsstring: "USVString", # Again, that's what it is in spec terms
         Types.object: "object",
-        Types.void: "void",
+        Types.undefined: "undefined",
         Types.ArrayBuffer: "ArrayBuffer",
         Types.ArrayBufferView: "ArrayBufferView",
         Types.Int8Array: "Int8Array",
@@ -3456,8 +3456,8 @@ class IDLBuiltinType(IDLType):
             return False
         if self.isObject():
             return other.isPrimitive() or other.isString() or other.isEnum()
-        if self.isVoid():
-            return not other.isVoid()
+        if self.isUndefined():
+            return not other.isUndefined()
         # Not much else we could be!
         assert self.isSpiderMonkeyInterface()
         # Like interfaces, but we know we're not a callback
@@ -3591,9 +3591,9 @@ BuiltinTypes = {
     IDLBuiltinType.Types.object:
         IDLBuiltinType(BuiltinLocation("<builtin type>"), "Object",
                        IDLBuiltinType.Types.object),
-    IDLBuiltinType.Types.void:
-        IDLBuiltinType(BuiltinLocation("<builtin type>"), "Void",
-                       IDLBuiltinType.Types.void),
+    IDLBuiltinType.Types.undefined:
+        IDLBuiltinType(BuiltinLocation("<builtin type>"), "Undefined",
+                       IDLBuiltinType.Types.undefined),
     IDLBuiltinType.Types.ArrayBuffer:
         IDLBuiltinType(BuiltinLocation("<builtin type>"), "ArrayBuffer",
                        IDLBuiltinType.Types.ArrayBuffer),
@@ -4196,9 +4196,9 @@ class IDLIterable(IDLMaplikeOrSetlikeOrIterableBase):
         self.addMethod("values", members, False, self.iteratorType,
                        affectsNothing=True, newObject=True)
 
-        # void forEach(callback(valueType, keyType), optional any thisArg)
+        # undefined forEach(callback(valueType, keyType), optional any thisArg)
         self.addMethod("forEach", members, False,
-                       BuiltinTypes[IDLBuiltinType.Types.void],
+                       BuiltinTypes[IDLBuiltinType.Types.undefined],
                        self.getForEachArguments())
 
     def isValueIterator(self):
@@ -4256,8 +4256,8 @@ class IDLMaplikeOrSetlike(IDLMaplikeOrSetlikeOrIterableBase):
         self.addMethod("values", members, False, BuiltinTypes[IDLBuiltinType.Types.object],
                        affectsNothing=True, isIteratorAlias=self.isSetlike())
 
-        # void forEach(callback(valueType, keyType), thisVal)
-        self.addMethod("forEach", members, False, BuiltinTypes[IDLBuiltinType.Types.void],
+        # undefined forEach(callback(valueType, keyType), thisVal)
+        self.addMethod("forEach", members, False, BuiltinTypes[IDLBuiltinType.Types.undefined],
                        self.getForEachArguments())
 
         def getKeyArg():
@@ -4270,8 +4270,8 @@ class IDLMaplikeOrSetlike(IDLMaplikeOrSetlikeOrIterableBase):
                        [getKeyArg()], isPure=True)
 
         if not self.readonly:
-            # void clear()
-            self.addMethod("clear", members, True, BuiltinTypes[IDLBuiltinType.Types.void],
+            # undefined clear()
+            self.addMethod("clear", members, True, BuiltinTypes[IDLBuiltinType.Types.undefined],
                            [])
             # boolean delete(keyType key)
             self.addMethod("delete", members, True,
@@ -4280,8 +4280,8 @@ class IDLMaplikeOrSetlike(IDLMaplikeOrSetlikeOrIterableBase):
         # Always generate underscored functions (e.g. __add, __clear) for js
         # implemented interfaces as convenience functions.
         if isJSImplemented:
-            # void clear()
-            self.addMethod("clear", members, True, BuiltinTypes[IDLBuiltinType.Types.void],
+            # undefined clear()
+            self.addMethod("clear", members, True, BuiltinTypes[IDLBuiltinType.Types.undefined],
                            [], chromeOnly=True)
             # boolean delete(keyType key)
             self.addMethod("delete", members, True,
@@ -5119,7 +5119,7 @@ class IDLMethod(IDLInterfaceMember, IDLScope):
             assert (arguments[0].type == BuiltinTypes[IDLBuiltinType.Types.domstring] or
                     arguments[0].type == BuiltinTypes[IDLBuiltinType.Types.unsigned_long])
             assert not arguments[0].optional and not arguments[0].variadic
-            assert not self._getter or not overload.returnType.isVoid()
+            assert not self._getter or not overload.returnType.isUndefined()
 
         if self._setter:
             assert len(self._overloads) == 1
@@ -5480,8 +5480,8 @@ class IDLMethod(IDLInterfaceMember, IDLScope):
             # This is called before we've done overload resolution
             overloads = self._overloads
             assert len(overloads) == 1
-            if not overloads[0].returnType.isVoid():
-                raise WebIDLError("[LenientFloat] used on a non-void method",
+            if not overloads[0].returnType.isUndefined():
+                raise WebIDLError("[LenientFloat] used on a non-undefined returning method",
                                   [attr.location, self.location])
             if not overloads[0].includesRestrictedFloatArgument():
                 raise WebIDLError("[LenientFloat] used on an operation with no "
@@ -5837,7 +5837,7 @@ class Tokenizer(object):
         "record": "RECORD",
         "short": "SHORT",
         "unsigned": "UNSIGNED",
-        "void": "VOID",
+        "undefined": "UNDEFINED",
         ":": "COLON",
         ";": "SEMICOLON",
         "{": "LBRACE",
@@ -6722,8 +6722,8 @@ class Parser(Tokenizer):
                                    "optional" if arguments[0].optional else "variadic"),
                                   [arguments[0].location])
         if getter:
-            if returnType.isVoid():
-                raise WebIDLError("getter cannot have void return type",
+            if returnType.isUndefined():
+                raise WebIDLError("getter cannot have undefined return type",
                                   [self.getLocation(p, 2)])
         if setter:
             if len(arguments) != 2:
@@ -7103,7 +7103,7 @@ class Parser(Tokenizer):
                   | SYMBOL
                   | TRUE
                   | UNSIGNED
-                  | VOID
+                  | UNDEFINED
                   | ArgumentNameKeyword
         """
         pass
@@ -7145,7 +7145,7 @@ class Parser(Tokenizer):
         """
         p[0] = BuiltinTypes[IDLBuiltinType.Types.any]
 
-    # Note: Promise<void> is allowed, so we want to parametrize on ReturnType,
+    # Note: Promise<undefined> is allowed, so we want to parametrize on ReturnType,
     # not Type.  Promise types can't be null, hence no "Null" in there.
     def p_SingleTypePromiseType(self, p):
         """
@@ -7413,11 +7413,11 @@ class Parser(Tokenizer):
         """
         p[0] = p[1]
 
-    def p_ReturnTypeVoid(self, p):
+    def p_ReturnTypeUndefined(self, p):
         """
-            ReturnType : VOID
+            ReturnType : UNDEFINED
         """
-        p[0] = BuiltinTypes[IDLBuiltinType.Types.void]
+        p[0] = BuiltinTypes[IDLBuiltinType.Types.undefined]
 
     def p_ScopedName(self, p):
         """
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_argument_identifier_conflicts.py b/components/script/dom/bindings/codegen/parser/tests/test_argument_identifier_conflicts.py
index eb1f6d3c92e..9ae85531fa3 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_argument_identifier_conflicts.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_argument_identifier_conflicts.py
@@ -3,7 +3,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface ArgumentIdentifierConflict {
-              void foo(boolean arg1, boolean arg1);
+              undefined foo(boolean arg1, boolean arg1);
             };
         """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_argument_keywords.py b/components/script/dom/bindings/codegen/parser/tests/test_argument_keywords.py
index e190f617e26..2b29658d678 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_argument_keywords.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_argument_keywords.py
@@ -1,7 +1,7 @@
 def WebIDLTest(parser, harness):
     parser.parse("""
     interface Foo {
-      void foo(object constructor);
+      undefined foo(object constructor);
     };
     """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_argument_novoid.py b/components/script/dom/bindings/codegen/parser/tests/test_argument_novoid.py
index ef8c2229aed..42e0776e677 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_argument_novoid.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_argument_novoid.py
@@ -2,8 +2,8 @@ def WebIDLTest(parser, harness):
     threw = False
     try:
         parser.parse("""
-            interface VoidArgument1 {
-              void foo(void arg2);
+            interface UndefinedArgument1 {
+              undefined foo(undefined arg2);
             };
         """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_arraybuffer.py b/components/script/dom/bindings/codegen/parser/tests/test_arraybuffer.py
index 4a96c0ff512..7020db59f3e 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_arraybuffer.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_arraybuffer.py
@@ -4,37 +4,37 @@ def WebIDLTest(parser, harness):
     parser.parse("""
         interface TestArrayBuffer {
           attribute ArrayBuffer bufferAttr;
-          void bufferMethod(ArrayBuffer arg1, ArrayBuffer? arg2, sequence<ArrayBuffer> arg3);
+          undefined bufferMethod(ArrayBuffer arg1, ArrayBuffer? arg2, sequence<ArrayBuffer> arg3);
 
           attribute ArrayBufferView viewAttr;
-          void viewMethod(ArrayBufferView arg1, ArrayBufferView? arg2, sequence<ArrayBufferView> arg3);
+          undefined viewMethod(ArrayBufferView arg1, ArrayBufferView? arg2, sequence<ArrayBufferView> arg3);
 
           attribute Int8Array int8ArrayAttr;
-          void int8ArrayMethod(Int8Array arg1, Int8Array? arg2, sequence<Int8Array> arg3);
+          undefined int8ArrayMethod(Int8Array arg1, Int8Array? arg2, sequence<Int8Array> arg3);
 
           attribute Uint8Array uint8ArrayAttr;
-          void uint8ArrayMethod(Uint8Array arg1, Uint8Array? arg2, sequence<Uint8Array> arg3);
+          undefined uint8ArrayMethod(Uint8Array arg1, Uint8Array? arg2, sequence<Uint8Array> arg3);
 
           attribute Uint8ClampedArray uint8ClampedArrayAttr;
-          void uint8ClampedArrayMethod(Uint8ClampedArray arg1, Uint8ClampedArray? arg2, sequence<Uint8ClampedArray> arg3);
+          undefined uint8ClampedArrayMethod(Uint8ClampedArray arg1, Uint8ClampedArray? arg2, sequence<Uint8ClampedArray> arg3);
 
           attribute Int16Array int16ArrayAttr;
-          void int16ArrayMethod(Int16Array arg1, Int16Array? arg2, sequence<Int16Array> arg3);
+          undefined int16ArrayMethod(Int16Array arg1, Int16Array? arg2, sequence<Int16Array> arg3);
 
           attribute Uint16Array uint16ArrayAttr;
-          void uint16ArrayMethod(Uint16Array arg1, Uint16Array? arg2, sequence<Uint16Array> arg3);
+          undefined uint16ArrayMethod(Uint16Array arg1, Uint16Array? arg2, sequence<Uint16Array> arg3);
 
           attribute Int32Array int32ArrayAttr;
-          void int32ArrayMethod(Int32Array arg1, Int32Array? arg2, sequence<Int32Array> arg3);
+          undefined int32ArrayMethod(Int32Array arg1, Int32Array? arg2, sequence<Int32Array> arg3);
 
           attribute Uint32Array uint32ArrayAttr;
-          void uint32ArrayMethod(Uint32Array arg1, Uint32Array? arg2, sequence<Uint32Array> arg3);
+          undefined uint32ArrayMethod(Uint32Array arg1, Uint32Array? arg2, sequence<Uint32Array> arg3);
 
           attribute Float32Array float32ArrayAttr;
-          void float32ArrayMethod(Float32Array arg1, Float32Array? arg2, sequence<Float32Array> arg3);
+          undefined float32ArrayMethod(Float32Array arg1, Float32Array? arg2, sequence<Float32Array> arg3);
 
           attribute Float64Array float64ArrayAttr;
-          void float64ArrayMethod(Float64Array arg1, Float64Array? arg2, sequence<Float64Array> arg3);
+          undefined float64ArrayMethod(Float64Array arg1, Float64Array? arg2, sequence<Float64Array> arg3);
         };
     """)
 
@@ -55,7 +55,7 @@ def WebIDLTest(parser, harness):
         harness.ok(attr.type.isSpiderMonkeyInterface(), "Should test as a js interface")
 
         (retType, arguments) = method.signatures()[0]
-        harness.ok(retType.isVoid(), "Should have a void return type")
+        harness.ok(retType.isUndefined(), "Should have a undefined return type")
         harness.check(len(arguments), 3, "Expect 3 arguments")
 
         harness.check(str(arguments[0].type), t,  "Expect an ArrayBuffer type")
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_attributes_on_types.py b/components/script/dom/bindings/codegen/parser/tests/test_attributes_on_types.py
index ff08791d16f..9ba39018c77 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_attributes_on_types.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_attributes_on_types.py
@@ -20,16 +20,16 @@ def WebIDLTest(parser, harness):
                 attribute [EnforceRange] long foo;
                 attribute [Clamp] long bar;
                 attribute [TreatNullAs=EmptyString] DOMString baz;
-                void method([EnforceRange] long foo, [Clamp] long bar,
+                undefined method([EnforceRange] long foo, [Clamp] long bar,
                             [TreatNullAs=EmptyString] DOMString baz);
-                void method2(optional [EnforceRange] long foo, optional [Clamp] long bar,
+                undefined method2(optional [EnforceRange] long foo, optional [Clamp] long bar,
                              optional [TreatNullAs=EmptyString] DOMString baz);
             };
             interface C {
                 attribute [EnforceRange] long? foo;
                 attribute [Clamp] long? bar;
-                void method([EnforceRange] long? foo, [Clamp] long? bar);
-                void method2(optional [EnforceRange] long? foo, optional [Clamp] long? bar);
+                undefined method([EnforceRange] long? foo, [Clamp] long? bar);
+                undefined method2(optional [EnforceRange] long? foo, optional [Clamp] long? bar);
             };
             interface Setlike {
                 setlike<[Clamp] long>;
@@ -98,13 +98,13 @@ def WebIDLTest(parser, harness):
             interface B {
                 attribute Foo typedefFoo;
                 attribute [AllowShared] ArrayBufferView foo;
-                void method([AllowShared] ArrayBufferView foo);
-                void method2(optional [AllowShared] ArrayBufferView foo);
+                undefined method([AllowShared] ArrayBufferView foo);
+                undefined method2(optional [AllowShared] ArrayBufferView foo);
             };
             interface C {
                 attribute [AllowShared] ArrayBufferView? foo;
-                void method([AllowShared] ArrayBufferView? foo);
-                void method2(optional [AllowShared] ArrayBufferView? foo);
+                undefined method([AllowShared] ArrayBufferView? foo);
+                undefined method2(optional [AllowShared] ArrayBufferView? foo);
             };
             interface Setlike {
                 setlike<[AllowShared] ArrayBufferView>;
@@ -154,7 +154,7 @@ def WebIDLTest(parser, harness):
         """),
         ("optional arguments", """
             interface Foo {
-                void foo(%s optional %s foo);
+                undefined foo(%s optional %s foo);
             };
         """),
         ("typedefs", """
@@ -189,7 +189,7 @@ def WebIDLTest(parser, harness):
         """),
         ("partial interface","""
             interface Foo {
-              void foo();
+              undefined foo();
             };
             %s
             partial interface Foo {
@@ -210,7 +210,7 @@ def WebIDLTest(parser, harness):
         """),
         ("partial namespace","""
             namespace Foo {
-              void foo();
+              undefined foo();
             };
             %s
             partial namespace Foo {
@@ -387,7 +387,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface Foo {
-               void foo([Clamp] Bar arg);
+               undefined foo([Clamp] Bar arg);
             };
             typedef long Bar;
         """)
@@ -403,7 +403,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface Foo {
-               void foo(Bar arg);
+               undefined foo(Bar arg);
             };
             typedef [Clamp] long Bar;
         """)
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_bytestring.py b/components/script/dom/bindings/codegen/parser/tests/test_bytestring.py
index fa83e9e2d57..51289f5db6e 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_bytestring.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_bytestring.py
@@ -60,8 +60,8 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface OptionalByteString {
-              void passByteString(optional ByteString arg = "hello");
-              };
+              undefined passByteString(optional ByteString arg = "hello");
+            };
         """)
         results2 = parser.finish();
     except WebIDL.WebIDLError as e:
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_callback_interface.py b/components/script/dom/bindings/codegen/parser/tests/test_callback_interface.py
index e4789dae168..34813bcab99 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_callback_interface.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_callback_interface.py
@@ -48,31 +48,31 @@ def WebIDLTest(parser, harness):
     parser = parser.reset()
     parser.parse("""
         callback interface TestCallbackInterface1 {
-          void foo();
+          undefined foo();
         };
         callback interface TestCallbackInterface2 {
-          void foo(DOMString arg);
-          void foo(TestCallbackInterface1 arg);
+          undefined foo(DOMString arg);
+          undefined foo(TestCallbackInterface1 arg);
         };
         callback interface TestCallbackInterface3 {
-          void foo(DOMString arg);
-          void foo(TestCallbackInterface1 arg);
-          static void bar();
+          undefined foo(DOMString arg);
+          undefined foo(TestCallbackInterface1 arg);
+          static undefined bar();
         };
         callback interface TestCallbackInterface4 {
-          void foo(DOMString arg);
-          void foo(TestCallbackInterface1 arg);
-          static void bar();
+          undefined foo(DOMString arg);
+          undefined foo(TestCallbackInterface1 arg);
+          static undefined bar();
           const long baz = 5;
         };
         callback interface TestCallbackInterface5 {
           static attribute boolean bool;
-          void foo();
+          undefined foo();
         };
         callback interface TestCallbackInterface6 {
-          void foo(DOMString arg);
-          void foo(TestCallbackInterface1 arg);
-          void bar();
+          undefined foo(DOMString arg);
+          undefined foo(TestCallbackInterface1 arg);
+          undefined bar();
         };
         callback interface TestCallbackInterface7 {
           static attribute boolean bool;
@@ -81,10 +81,10 @@ def WebIDLTest(parser, harness):
           attribute boolean bool;
         };
         callback interface TestCallbackInterface9 : TestCallbackInterface1 {
-          void foo();
+          undefined foo();
         };
         callback interface TestCallbackInterface10 : TestCallbackInterface1 {
-          void bar();
+          undefined bar();
         };
     """)
     results = parser.finish()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_cereactions.py b/components/script/dom/bindings/codegen/parser/tests/test_cereactions.py
index f726907c2fc..ebc688bfd9c 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_cereactions.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_cereactions.py
@@ -3,7 +3,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface Foo {
-              [CEReactions(DOMString a)] void foo(boolean arg2);
+              [CEReactions(DOMString a)] undefined foo(boolean arg2);
             };
         """)
 
@@ -47,7 +47,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface Foo {
-              [CEReactions] void foo(boolean arg2);
+              [CEReactions] undefined foo(boolean arg2);
             };
         """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_dictionary.py b/components/script/dom/bindings/codegen/parser/tests/test_dictionary.py
index 3cad3022389..dcdc43d5c47 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_dictionary.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_dictionary.py
@@ -158,7 +158,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(A arg);
+              undefined doFoo(A arg);
             };
         """)
         results = parser.finish()
@@ -174,7 +174,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional A arg);
+              undefined doFoo(optional A arg);
             };
         """)
         results = parser.finish()
@@ -190,7 +190,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo((A or DOMString) arg);
+              undefined doFoo((A or DOMString) arg);
             };
         """)
         results = parser.finish()
@@ -207,7 +207,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional (A or DOMString) arg);
+              undefined doFoo(optional (A or DOMString) arg);
             };
         """)
         results = parser.finish()
@@ -224,7 +224,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(A arg1, optional long arg2);
+              undefined doFoo(A arg1, optional long arg2);
             };
         """)
         results = parser.finish()
@@ -240,7 +240,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional A arg1, optional long arg2);
+              undefined doFoo(optional A arg1, optional long arg2);
             };
         """)
         results = parser.finish()
@@ -256,7 +256,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(A arg1, optional long arg2, long arg3);
+              undefined doFoo(A arg1, optional long arg2, long arg3);
             };
         """)
         results = parser.finish()
@@ -273,7 +273,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo((A or DOMString) arg1, optional long arg2);
+              undefined doFoo((A or DOMString) arg1, optional long arg2);
             };
         """)
         results = parser.finish()
@@ -291,7 +291,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional (A or DOMString) arg1, optional long arg2);
+              undefined doFoo(optional (A or DOMString) arg1, optional long arg2);
             };
         """)
         results = parser.finish()
@@ -307,7 +307,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(A arg1, long arg2);
+              undefined doFoo(A arg1, long arg2);
             };
         """)
     results = parser.finish()
@@ -320,7 +320,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional A? arg1 = {});
+              undefined doFoo(optional A? arg1 = {});
             };
         """)
         results = parser.finish()
@@ -339,7 +339,7 @@ def WebIDLTest(parser, harness):
               required long x;
             };
             interface X {
-              void doFoo(A? arg1);
+              undefined doFoo(A? arg1);
             };
         """)
         results = parser.finish()
@@ -358,7 +358,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional (A or long)? arg1 = {});
+              undefined doFoo(optional (A or long)? arg1 = {});
             };
         """)
         results = parser.finish()
@@ -378,7 +378,7 @@ def WebIDLTest(parser, harness):
               required long x;
             };
             interface X {
-              void doFoo((A or long)? arg1);
+              undefined doFoo((A or long)? arg1);
             };
         """)
         results = parser.finish()
@@ -397,7 +397,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(sequence<A?> arg1);
+              undefined doFoo(sequence<A?> arg1);
             };
         """)
         results = parser.finish()
@@ -414,7 +414,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional (A or long?) arg1);
+              undefined doFoo(optional (A or long?) arg1);
             };
         """)
         results = parser.finish()
@@ -430,7 +430,7 @@ def WebIDLTest(parser, harness):
             dictionary A {
             };
             interface X {
-              void doFoo(optional (long? or A) arg1);
+              undefined doFoo(optional (long? or A) arg1);
             };
         """)
         results = parser.finish()
@@ -455,7 +455,7 @@ def WebIDLTest(parser, harness):
         dictionary A {
         };
         interface X {
-          void doFoo(optional A arg = {});
+          undefined doFoo(optional A arg = {});
         };
     """)
     results = parser.finish()
@@ -466,7 +466,7 @@ def WebIDLTest(parser, harness):
         dictionary A {
         };
         interface X {
-          void doFoo(optional (A or DOMString) arg = {});
+          undefined doFoo(optional (A or DOMString) arg = {});
         };
     """)
     results = parser.finish()
@@ -477,7 +477,7 @@ def WebIDLTest(parser, harness):
         dictionary A {
         };
         interface X {
-          void doFoo(optional (A or DOMString) arg = "abc");
+          undefined doFoo(optional (A or DOMString) arg = "abc");
         };
     """)
     results = parser.finish()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_distinguishability.py b/components/script/dom/bindings/codegen/parser/tests/test_distinguishability.py
index 505b36468d6..1fa12832d7f 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_distinguishability.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_distinguishability.py
@@ -12,10 +12,10 @@ def WebIDLTest(parser, harness):
       };
       interface Bar {
         // Bit of a pain to get things that have dictionary types
-        void passDict(Dict arg);
-        void passFoo(Foo arg);
-        void passNullableUnion((object? or DOMString) arg);
-        void passNullable(Foo? arg);
+        undefined passDict(Dict arg);
+        undefined passFoo(Foo arg);
+        undefined passNullableUnion((object? or DOMString) arg);
+        undefined passNullable(Foo? arg);
       };
     """)
     results = parser.finish()
@@ -56,13 +56,13 @@ def WebIDLTest(parser, harness):
     parser = parser.reset()
     parser.parse("""
       interface TestIface {
-        void passKid(Kid arg);
-        void passParent(Parent arg);
-        void passGrandparent(Grandparent arg);
-        void passUnrelated1(Unrelated1 arg);
-        void passUnrelated2(Unrelated2 arg);
-        void passArrayBuffer(ArrayBuffer arg);
-        void passArrayBuffer(ArrayBufferView arg);
+        undefined passKid(Kid arg);
+        undefined passParent(Parent arg);
+        undefined passGrandparent(Grandparent arg);
+        undefined passUnrelated1(Unrelated1 arg);
+        undefined passUnrelated2(Unrelated2 arg);
+        undefined passArrayBuffer(ArrayBuffer arg);
+        undefined passArrayBuffer(ArrayBufferView arg);
       };
 
       interface Kid : Parent {};
@@ -97,10 +97,10 @@ def WebIDLTest(parser, harness):
     parser.parse("""
       interface Dummy {};
       interface TestIface {
-        void method(long arg1, TestIface arg2);
-        void method(long arg1, long arg2);
-        void method(long arg1, Dummy arg2);
-        void method(DOMString arg1, DOMString arg2, DOMString arg3);
+        undefined method(long arg1, TestIface arg2);
+        undefined method(long arg1, long arg2);
+        undefined method(long arg1, Dummy arg2);
+        undefined method(DOMString arg1, DOMString arg2, DOMString arg3);
       };
     """)
     results = parser.finish()
@@ -115,10 +115,10 @@ def WebIDLTest(parser, harness):
         parser.parse("""
           interface Dummy {};
           interface TestIface {
-            void method(long arg1, TestIface arg2);
-            void method(long arg1, long arg2);
-            void method(any arg1,  Dummy arg2);
-            void method(DOMString arg1, DOMString arg2, DOMString arg3);
+            undefined method(long arg1, TestIface arg2);
+            undefined method(long arg1, long arg2);
+            undefined method(any arg1,  Dummy arg2);
+            undefined method(DOMString arg1, DOMString arg2, DOMString arg3);
           };
         """)
         results = parser.finish()
@@ -135,10 +135,10 @@ def WebIDLTest(parser, harness):
         parser.parse("""
           interface Dummy {};
           interface TestIface {
-            void method(long arg1, TestIface arg2);
-            void method(long arg1, long arg2);
-            void method(any arg1,  DOMString arg2);
-            void method(DOMString arg1, DOMString arg2, DOMString arg3);
+            undefined method(long arg1, TestIface arg2);
+            undefined method(long arg1, long arg2);
+            undefined method(any arg1,  DOMString arg2);
+            undefined method(DOMString arg1, DOMString arg2, DOMString arg3);
           };
         """)
         results = parser.finish()
@@ -272,7 +272,7 @@ def WebIDLTest(parser, harness):
           };
         """
         methodTemplate = """
-            void myMethod(%s arg);"""
+            undefined myMethod(%s arg);"""
         methods = (methodTemplate % type1) + (methodTemplate % type2)
         idl = idlTemplate % methods
         parser = parser.reset()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_empty_sequence_default_value.py b/components/script/dom/bindings/codegen/parser/tests/test_empty_sequence_default_value.py
index a713266c88e..5f04c6ae751 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_empty_sequence_default_value.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_empty_sequence_default_value.py
@@ -19,7 +19,7 @@ def WebIDLTest(parser, harness):
 
     parser.parse("""
         interface X {
-          void foo(optional sequence<long> arg = []);
+          undefined foo(optional sequence<long> arg = []);
         };
     """)
     results = parser.finish();
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_enum.py b/components/script/dom/bindings/codegen/parser/tests/test_enum.py
index 86228939181..c5617ead99a 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_enum.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_enum.py
@@ -71,7 +71,7 @@ def WebIDLTest(parser, harness):
             "c"
           };
           interface TestInterface {
-            void foo(optional Enum e = "d");
+            undefined foo(optional Enum e = "d");
           };
         """)
         results = parser.finish()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_exposed_extended_attribute.py b/components/script/dom/bindings/codegen/parser/tests/test_exposed_extended_attribute.py
index e0241a56426..39993eaeae5 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_exposed_extended_attribute.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_exposed_extended_attribute.py
@@ -8,7 +8,7 @@ def WebIDLTest(parser, harness):
 
       [Exposed=(Foo,Bar1)]
       interface Iface {
-        void method1();
+        undefined method1();
 
         [Exposed=Bar1]
         readonly attribute any attr;
@@ -16,7 +16,7 @@ def WebIDLTest(parser, harness):
 
       [Exposed=Foo]
       partial interface Iface {
-        void method2();
+        undefined method2();
       };
     """)
 
@@ -57,7 +57,7 @@ def WebIDLTest(parser, harness):
 
       [Exposed=Foo]
       interface Iface2 {
-        void method3();
+        undefined method3();
       };
     """)
     results = parser.finish()
@@ -87,12 +87,12 @@ def WebIDLTest(parser, harness):
 
       [Exposed=Foo]
       interface Iface3 {
-        void method4();
+        undefined method4();
       };
 
       [Exposed=(Foo,Bar1)]
       interface mixin Mixin {
-        void method5();
+        undefined method5();
       };
 
       Iface3 includes Mixin;
@@ -152,7 +152,7 @@ def WebIDLTest(parser, harness):
         parser.parse("""
             interface Bar {
               [Exposed=Foo]
-              void operation();
+              undefined operation();
             };
         """)
 
@@ -188,7 +188,7 @@ def WebIDLTest(parser, harness):
             [Exposed=Foo]
             interface Baz {
               [Exposed=Bar]
-              void method();
+              undefined method();
             };
         """)
 
@@ -205,12 +205,12 @@ def WebIDLTest(parser, harness):
 
         [Exposed=Foo]
         interface Baz {
-          void method();
+          undefined method();
         };
 
         [Exposed=Bar]
         interface mixin Mixin {
-          void otherMethod();
+          undefined otherMethod();
         };
 
         Baz includes Mixin;
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_extended_attributes.py b/components/script/dom/bindings/codegen/parser/tests/test_extended_attributes.py
index 144c945bc10..66909f322c2 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_extended_attributes.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_extended_attributes.py
@@ -48,8 +48,8 @@ def WebIDLTest(parser, harness):
     parser = parser.reset()
     parser.parse("""
         interface TestClamp {
-          void testClamp([Clamp] long foo);
-          void testNotClamp(long foo);
+          undefined testClamp([Clamp] long foo);
+          undefined testNotClamp(long foo);
         };
     """)
 
@@ -66,7 +66,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface TestClamp2 {
-              void testClamp([Clamp=something] long foo);
+              undefined testClamp([Clamp=something] long foo);
             };
         """)
         results = parser.finish()
@@ -78,8 +78,8 @@ def WebIDLTest(parser, harness):
     parser = parser.reset()
     parser.parse("""
         interface TestEnforceRange {
-          void testEnforceRange([EnforceRange] long foo);
-          void testNotEnforceRange(long foo);
+          undefined testEnforceRange([EnforceRange] long foo);
+          undefined testNotEnforceRange(long foo);
         };
     """)
 
@@ -96,7 +96,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface TestEnforceRange2 {
-              void testEnforceRange([EnforceRange=something] long foo);
+              undefined testEnforceRange([EnforceRange=something] long foo);
             };
         """)
         results = parser.finish()
@@ -104,4 +104,4 @@ def WebIDLTest(parser, harness):
         threw = True
 
     harness.ok(threw, "[EnforceRange] must take no arguments")
-    
+
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_float_types.py b/components/script/dom/bindings/codegen/parser/tests/test_float_types.py
index b7325cf9d26..8fbe9394042 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_float_types.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_float_types.py
@@ -14,23 +14,23 @@ def WebIDLTest(parser, harness):
           [LenientFloat]
           attribute double ld;
 
-          void m1(float arg1, double arg2, float? arg3, double? arg4,
+          undefined m1(float arg1, double arg2, float? arg3, double? arg4,
                   myFloat arg5, unrestricted float arg6,
                   unrestricted double arg7, unrestricted float? arg8,
                   unrestricted double? arg9, myUnrestrictedFloat arg10);
           [LenientFloat]
-          void m2(float arg1, double arg2, float? arg3, double? arg4,
+          undefined m2(float arg1, double arg2, float? arg3, double? arg4,
                   myFloat arg5, unrestricted float arg6,
                   unrestricted double arg7, unrestricted float? arg8,
                   unrestricted double? arg9, myUnrestrictedFloat arg10);
           [LenientFloat]
-          void m3(float arg);
+          undefined m3(float arg);
           [LenientFloat]
-          void m4(double arg);
+          undefined m4(double arg);
           [LenientFloat]
-          void m5((float or FloatTypes) arg);
+          undefined m5((float or FloatTypes) arg);
           [LenientFloat]
-          void m6(sequence<float> arg);
+          undefined m6(sequence<float> arg);
         };
     """)
 
@@ -70,7 +70,7 @@ def WebIDLTest(parser, harness):
         """)
     except Exception as x:
         threw = True
-    harness.ok(threw, "[LenientFloat] only allowed on void methods")
+    harness.ok(threw, "[LenientFloat] only allowed on undefined-retuning methods")
 
     parser = parser.reset()
     threw = False
@@ -78,7 +78,7 @@ def WebIDLTest(parser, harness):
         parser.parse("""
             interface FloatTypes {
               [LenientFloat]
-              void m(unrestricted float arg);
+              undefined m(unrestricted float arg);
             };
         """)
     except Exception as x:
@@ -91,7 +91,7 @@ def WebIDLTest(parser, harness):
         parser.parse("""
             interface FloatTypes {
               [LenientFloat]
-              void m(sequence<unrestricted float> arg);
+              undefined m(sequence<unrestricted float> arg);
             };
         """)
     except Exception as x:
@@ -104,7 +104,7 @@ def WebIDLTest(parser, harness):
         parser.parse("""
             interface FloatTypes {
               [LenientFloat]
-              void m((unrestricted float or FloatTypes) arg);
+              undefined m((unrestricted float or FloatTypes) arg);
             };
         """)
     except Exception as x:
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_global_extended_attr.py b/components/script/dom/bindings/codegen/parser/tests/test_global_extended_attr.py
index 28b79642d86..3958f8ce104 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_global_extended_attr.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_global_extended_attr.py
@@ -22,7 +22,7 @@ def WebIDLTest(parser, harness):
           [Global, Exposed=Foo]
           interface Foo {
             getter any(DOMString name);
-            setter void(DOMString name, any arg);
+            setter undefined(DOMString name, any arg);
           };
         """)
         results = parser.finish()
@@ -40,7 +40,7 @@ def WebIDLTest(parser, harness):
           [Global, Exposed=Foo]
           interface Foo {
             getter any(DOMString name);
-            deleter void(DOMString name);
+            deleter undefined(DOMString name);
           };
         """)
         results = parser.finish()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_incomplete_parent.py b/components/script/dom/bindings/codegen/parser/tests/test_incomplete_parent.py
index 1f520a28e16..8f30c212d7b 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_incomplete_parent.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_incomplete_parent.py
@@ -3,7 +3,7 @@ import WebIDL
 def WebIDLTest(parser, harness):
     parser.parse("""
         interface TestIncompleteParent : NotYetDefined {
-          void foo();
+          undefined foo();
         };
 
         interface NotYetDefined : EvenHigherOnTheChain {
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_interface.py b/components/script/dom/bindings/codegen/parser/tests/test_interface.py
index 47db3ae4cc9..a10bcd9863d 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_interface.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_interface.py
@@ -32,7 +32,7 @@ def WebIDLTest(parser, harness):
 
         interface QNameDerived : QNameBase {
           attribute long long foo;
-          attribute byte bar;          
+          attribute byte bar;
         };
     """)
     results = parser.finish()
@@ -99,11 +99,11 @@ def WebIDLTest(parser, harness):
             constructor();
             constructor(long arg);
             readonly attribute boolean x;
-            void foo();
+            undefined foo();
         };
         partial interface A {
             readonly attribute boolean y;
-            void foo(long arg);
+            undefined foo(long arg);
         };
     """);
     results = parser.finish();
@@ -127,13 +127,13 @@ def WebIDLTest(parser, harness):
     parser.parse("""
         partial interface A {
             readonly attribute boolean y;
-            void foo(long arg);
+            undefined foo(long arg);
         };
         interface A {
             constructor();
             constructor(long arg);
             readonly attribute boolean x;
-            void foo();
+            undefined foo();
         };
     """);
     results = parser.finish();
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_interface_maplikesetlikeiterable.py b/components/script/dom/bindings/codegen/parser/tests/test_interface_maplikesetlikeiterable.py
index e070adee7e6..835212d2965 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_interface_maplikesetlikeiterable.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_interface_maplikesetlikeiterable.py
@@ -358,7 +358,7 @@ def WebIDLTest(parser, harness):
                        interface Foo1 {
                        %s;
                        [Throws]
-                       void %s(long test1, double test2, double test3);
+                       undefined %s(long test1, double test2, double test3);
                        };
                        """ % (likeMember, conflictName), expectedMembers)
         else:
@@ -367,14 +367,14 @@ def WebIDLTest(parser, harness):
                        interface Foo1 {
                        %s;
                        [Throws]
-                       void %s(long test1, double test2, double test3);
+                       undefined %s(long test1, double test2, double test3);
                        };
                        """ % (likeMember, conflictName))
         # Inherited conflicting methods should ALWAYS fail
         shouldFail("Conflicting inherited method: %s and %s" % (likeMember, conflictName),
                    """
                    interface Foo1 {
-                   void %s(long test1, double test2, double test3);
+                   undefined %s(long test1, double test2, double test3);
                    };
                    interface Foo2 : Foo1 {
                    %s;
@@ -384,7 +384,7 @@ def WebIDLTest(parser, harness):
                    """
                    interface Foo1 {
                    %s;
-                   static void %s(long test1, double test2, double test3);
+                   static undefined %s(long test1, double test2, double test3);
                    };
                    """ % (likeMember, conflictName))
         shouldFail("Conflicting attribute: %s and %s" % (likeMember, conflictName),
@@ -426,7 +426,7 @@ def WebIDLTest(parser, harness):
                maplike<long, long>;
                };
                interface Foo2 : Foo1 {
-               void entries();
+               undefined entries();
                };
                """, mapRWMembers, numProductions=2)
 
@@ -438,7 +438,7 @@ def WebIDLTest(parser, harness):
                interface Foo2 : Foo1 {
                };
                interface Foo3 : Foo2 {
-               void entries();
+               undefined entries();
                };
                """, mapRWMembers, numProductions=3)
 
@@ -448,7 +448,7 @@ def WebIDLTest(parser, harness):
                maplike<long, long>;
                };
                interface mixin Foo2 {
-               void entries();
+               undefined entries();
                };
                Foo1 includes Foo2;
                """)
@@ -459,7 +459,7 @@ def WebIDLTest(parser, harness):
                maplike<long, long>;
                };
                interface mixin Foo2 {
-               void entries();
+               undefined entries();
                };
                interface Foo3 : Foo1 {
                };
@@ -469,7 +469,7 @@ def WebIDLTest(parser, harness):
     shouldFail("Inheritance of name collision with child maplike/setlike",
                """
                interface Foo1 {
-               void entries();
+               undefined entries();
                };
                interface Foo2 : Foo1 {
                maplike<long, long>;
@@ -479,7 +479,7 @@ def WebIDLTest(parser, harness):
     shouldFail("Inheritance of multi-level name collision with child maplike/setlike",
                """
                interface Foo1 {
-               void entries();
+               undefined entries();
                };
                interface Foo2 : Foo1 {
                };
@@ -558,7 +558,7 @@ def WebIDLTest(parser, harness):
                maplike<long, long>;
                };
                interface Foo2 : Foo1 {
-               void clear();
+               undefined clear();
                };
                """, mapRWMembers, numProductions=2)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_interfacemixin.py b/components/script/dom/bindings/codegen/parser/tests/test_interfacemixin.py
index 477a9f37799..79cf1f6780d 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_interfacemixin.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_interfacemixin.py
@@ -30,11 +30,11 @@ def WebIDLTest(parser, harness):
     parser.parse("""
         interface mixin A {
             readonly attribute boolean x;
-            void foo();
+            undefined foo();
         };
         partial interface mixin A {
             readonly attribute boolean y;
-            void foo(long arg);
+            undefined foo(long arg);
         };
     """)
     results = parser.finish()
@@ -56,11 +56,11 @@ def WebIDLTest(parser, harness):
     parser.parse("""
         partial interface mixin A {
             readonly attribute boolean y;
-            void foo(long arg);
+            undefined foo(long arg);
         };
         interface mixin A {
             readonly attribute boolean x;
-            void foo();
+            undefined foo();
         };
     """)
     results = parser.finish()
@@ -212,7 +212,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface mixin A {
-                setter void (DOMString propertyName, double propertyValue);
+                setter undefined (DOMString propertyName, double propertyValue);
             };
         """)
         results = parser.finish()
@@ -226,7 +226,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface mixin A {
-                deleter void (DOMString propertyName);
+                deleter undefined (DOMString propertyName);
             };
         """)
         results = parser.finish()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_method.py b/components/script/dom/bindings/codegen/parser/tests/test_method.py
index 88ee874386c..ff1f087c861 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_method.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_method.py
@@ -3,17 +3,17 @@ import WebIDL
 def WebIDLTest(parser, harness):
     parser.parse("""
         interface TestMethods {
-          void basic();
-          static void basicStatic();
-          void basicWithSimpleArgs(boolean arg1, byte arg2, unsigned long arg3);
+          undefined basic();
+          static undefined basicStatic();
+          undefined basicWithSimpleArgs(boolean arg1, byte arg2, unsigned long arg3);
           boolean basicBoolean();
           static boolean basicStaticBoolean();
           boolean basicBooleanWithSimpleArgs(boolean arg1, byte arg2, unsigned long arg3);
-          void optionalArg(optional byte? arg1, optional sequence<byte> arg2);
-          void variadicArg(byte?... arg1);
+          undefined optionalArg(optional byte? arg1, optional sequence<byte> arg2);
+          undefined variadicArg(byte?... arg1);
           object getObject();
-          void setObject(object arg1);
-          void setAny(any arg1);
+          undefined setObject(object arg1);
+          undefined setAny(any arg1);
           float doFloats(float arg1);
         };
     """)
@@ -70,12 +70,12 @@ def WebIDLTest(parser, harness):
                 (QName, name, type, optional, variadic) = expectedArgs[i]
                 checkArgument(gotArgs[i], QName, name, type, optional, variadic)
 
-    checkMethod(methods[0], "::TestMethods::basic", "basic", [("Void", [])])
+    checkMethod(methods[0], "::TestMethods::basic", "basic", [("Undefined", [])])
     checkMethod(methods[1], "::TestMethods::basicStatic", "basicStatic",
-                [("Void", [])], static=True)
+                [("Undefined", [])], static=True)
     checkMethod(methods[2], "::TestMethods::basicWithSimpleArgs",
                 "basicWithSimpleArgs",
-       [("Void",
+       [("Undefined",
         [("::TestMethods::basicWithSimpleArgs::arg1", "arg1", "Boolean", False, False),
          ("::TestMethods::basicWithSimpleArgs::arg2", "arg2", "Byte", False, False),
          ("::TestMethods::basicWithSimpleArgs::arg3", "arg3", "UnsignedLong", False, False)])])
@@ -89,22 +89,22 @@ def WebIDLTest(parser, harness):
          ("::TestMethods::basicBooleanWithSimpleArgs::arg3", "arg3", "UnsignedLong", False, False)])])
     checkMethod(methods[6], "::TestMethods::optionalArg",
                 "optionalArg",
-       [("Void",
+       [("Undefined",
         [("::TestMethods::optionalArg::arg1", "arg1", "ByteOrNull", True, False),
          ("::TestMethods::optionalArg::arg2", "arg2", "ByteSequence", True, False)])])
     checkMethod(methods[7], "::TestMethods::variadicArg",
                 "variadicArg",
-       [("Void",
+       [("Undefined",
         [("::TestMethods::variadicArg::arg1", "arg1", "ByteOrNull", True, True)])])
     checkMethod(methods[8], "::TestMethods::getObject",
                 "getObject", [("Object", [])])
     checkMethod(methods[9], "::TestMethods::setObject",
                 "setObject",
-       [("Void",
+       [("Undefined",
         [("::TestMethods::setObject::arg1", "arg1", "Object", False, False)])])
     checkMethod(methods[10], "::TestMethods::setAny",
                 "setAny",
-       [("Void",
+       [("Undefined",
         [("::TestMethods::setAny::arg1", "arg1", "Any", False, False)])])
     checkMethod(methods[11], "::TestMethods::doFloats",
                 "doFloats",
@@ -116,7 +116,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
           interface A {
-            void foo(optional float bar = 1);
+            undefined foo(optional float bar = 1);
           };
         """)
         results = parser.finish()
@@ -129,7 +129,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
           interface A {
-            [GetterThrows] void foo();
+            [GetterThrows] undefined foo();
           };
         """)
         results = parser.finish()
@@ -142,7 +142,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
           interface A {
-            [SetterThrows] void foo();
+            [SetterThrows] undefined foo();
           };
         """)
         results = parser.finish()
@@ -155,7 +155,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
           interface A {
-            [Throw] void foo();
+            [Throw] undefined foo();
           };
         """)
         results = parser.finish()
@@ -168,7 +168,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
           interface A {
-            void __noSuchMethod__();
+            undefined __noSuchMethod__();
           };
         """)
         results = parser.finish()
@@ -182,9 +182,9 @@ def WebIDLTest(parser, harness):
         parser.parse("""
           interface A {
             [Throws, LenientFloat]
-            void foo(float myFloat);
+            undefined foo(float myFloat);
             [Throws]
-            void foo();
+            undefined foo();
           };
         """)
         results = parser.finish()
@@ -196,9 +196,9 @@ def WebIDLTest(parser, harness):
     parser.parse("""
       interface A {
         [Throws]
-        void foo();
+        undefined foo();
         [Throws, LenientFloat]
-        void foo(float myFloat);
+        undefined foo(float myFloat);
       };
     """)
     results = parser.finish()
@@ -213,9 +213,9 @@ def WebIDLTest(parser, harness):
         parser.parse("""
           interface A {
             [Throws, LenientFloat]
-            void foo(float myFloat);
+            undefined foo(float myFloat);
             [Throws]
-            void foo(float myFloat, float yourFloat);
+            undefined foo(float myFloat, float yourFloat);
           };
         """)
         results = parser.finish()
@@ -229,9 +229,9 @@ def WebIDLTest(parser, harness):
         parser.parse("""
           interface A {
             [Throws]
-            void foo(float myFloat, float yourFloat);
+            undefined foo(float myFloat, float yourFloat);
             [Throws, LenientFloat]
-            void foo(float myFloat);
+            undefined foo(float myFloat);
           };
         """)
         results = parser.finish()
@@ -245,9 +245,9 @@ def WebIDLTest(parser, harness):
         parser.parse("""
           interface A {
             [Throws, LenientFloat]
-            void foo(float myFloat);
+            undefined foo(float myFloat);
             [Throws, LenientFloat]
-            void foo(short myShort);
+            undefined foo(short myShort);
           };
         """)
         results = parser.finish()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_nullable_void.py b/components/script/dom/bindings/codegen/parser/tests/test_nullable_void.py
index 961ff825e9f..ebf841a5205 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_nullable_void.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_nullable_void.py
@@ -2,8 +2,8 @@ def WebIDLTest(parser, harness):
     threw = False
     try:
         parser.parse("""
-            interface NullableVoid {
-              void? foo();
+            interface NullableUndefined {
+              undefined? foo();
             };
         """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_optional_constraints.py b/components/script/dom/bindings/codegen/parser/tests/test_optional_constraints.py
index 6217465ce7d..ad012b633d5 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_optional_constraints.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_optional_constraints.py
@@ -3,7 +3,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface OptionalConstraints1 {
-              void foo(optional byte arg1, byte arg2);
+              undefined foo(optional byte arg1, byte arg2);
             };
         """)
 
@@ -18,7 +18,7 @@ def WebIDLTest(parser, harness):
     parser = parser.reset()
     parser.parse("""
         interface OptionalConstraints2 {
-          void foo(optional byte arg1 = 1, optional byte arg2 = 2,
+          undefined foo(optional byte arg1 = 1, optional byte arg2 = 2,
                    optional byte arg3, optional byte arg4 = 4,
                    optional byte arg5, optional byte arg6 = 9);
         };
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_overload.py b/components/script/dom/bindings/codegen/parser/tests/test_overload.py
index 3c680ad5233..8e02f64d6c9 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_overload.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_overload.py
@@ -3,16 +3,16 @@ import WebIDL
 def WebIDLTest(parser, harness):
     parser.parse("""
         interface TestOverloads {
-          void basic();
-          void basic(long arg1);
+          undefined basic();
+          undefined basic(long arg1);
           boolean abitharder(TestOverloads foo);
           boolean abitharder(boolean foo);
-          void abitharder(ArrayBuffer? foo);
-          void withVariadics(long... numbers);
-          void withVariadics(TestOverloads iface);
-          void withVariadics(long num, TestOverloads iface);
-          void optionalTest();
-          void optionalTest(optional long num1, long num2);
+          undefined abitharder(ArrayBuffer? foo);
+          undefined withVariadics(long... numbers);
+          undefined withVariadics(TestOverloads iface);
+          undefined withVariadics(long num, TestOverloads iface);
+          undefined optionalTest();
+          undefined optionalTest(optional long num1, long num2);
         };
     """)
 
@@ -37,11 +37,11 @@ def WebIDLTest(parser, harness):
 
     (retval, argumentSet) = signatures[0]
 
-    harness.check(str(retval), "Void", "Expect a void retval")
+    harness.check(str(retval), "Undefined", "Expect a undefined retval")
     harness.check(len(argumentSet), 0, "Expect an empty argument set")
 
     (retval, argumentSet) = signatures[1]
-    harness.check(str(retval), "Void", "Expect a void retval")
+    harness.check(str(retval), "Undefined", "Expect a undefined retval")
     harness.check(len(argumentSet), 1, "Expect an argument set with one argument")
 
     argument = argumentSet[0]
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_promise.py b/components/script/dom/bindings/codegen/parser/tests/test_promise.py
index 43c74029dc5..ef44a216d10 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_promise.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_promise.py
@@ -64,7 +64,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface A {
-              void foo(Promise<any>? arg);
+              undefined foo(Promise<any>? arg);
             };
         """)
         results = parser.finish();
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_record.py b/components/script/dom/bindings/codegen/parser/tests/test_record.py
index d50572caf07..3d83e249be8 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_record.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_record.py
@@ -4,7 +4,7 @@ def WebIDLTest(parser, harness):
     parser.parse("""
         dictionary Dict {};
         interface RecordArg {
-          void foo(record<DOMString, Dict> arg);
+          undefined foo(record<DOMString, Dict> arg);
         };
     """)
 
@@ -27,16 +27,16 @@ def WebIDLTest(parser, harness):
     threw = False
     try:
         parser.parse("""
-            interface RecordVoidArg {
-              void foo(record<DOMString, void> arg);
+            interface RecordUndefinedArg {
+              undefined foo(record<DOMString, undefined> arg);
             };
         """)
 
         results = parser.finish()
     except Exception as x:
         threw = True
-    harness.ok(threw, "Should have thrown because record can't have void as value type.")
- 
+    harness.ok(threw, "Should have thrown because record can't have undefined as value type.")
+
     parser = parser.reset()
     threw = False
     try:
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_securecontext_extended_attribute.py b/components/script/dom/bindings/codegen/parser/tests/test_securecontext_extended_attribute.py
index 442dba45d76..5af0c22803c 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_securecontext_extended_attribute.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_securecontext_extended_attribute.py
@@ -6,12 +6,12 @@ def WebIDLTest(parser, harness):
         interface TestSecureContextOnInterface {
           const octet TEST_CONSTANT = 0;
           readonly attribute byte testAttribute;
-          void testMethod(byte foo);
+          undefined testMethod(byte foo);
         };
         partial interface TestSecureContextOnInterface {
           const octet TEST_CONSTANT_2 = 0;
           readonly attribute byte testAttribute2;
-          void testMethod2(byte foo);
+          undefined testMethod2(byte foo);
         };
     """)
     results = parser.finish()
@@ -37,13 +37,13 @@ def WebIDLTest(parser, harness):
         partial interface TestSecureContextOnInterfaceAfterPartialInterface {
           const octet TEST_CONSTANT_2 = 0;
           readonly attribute byte testAttribute2;
-          void testMethod2(byte foo);
+          undefined testMethod2(byte foo);
         };
         [SecureContext]
         interface TestSecureContextOnInterfaceAfterPartialInterface {
           const octet TEST_CONSTANT = 0;
           readonly attribute byte testAttribute;
-          void testMethod(byte foo);
+          undefined testMethod(byte foo);
         };
      """)
     results = parser.finish()
@@ -68,13 +68,13 @@ def WebIDLTest(parser, harness):
         interface TestSecureContextOnPartialInterface {
           const octet TEST_CONSTANT = 0;
           readonly attribute byte testAttribute;
-          void testMethod(byte foo);
+          undefined testMethod(byte foo);
         };
         [SecureContext]
         partial interface TestSecureContextOnPartialInterface {
           const octet TEST_CONSTANT_2 = 0;
           readonly attribute byte testAttribute2;
-          void testMethod2(byte foo);
+          undefined testMethod2(byte foo);
         };
     """)
     results = parser.finish()
@@ -105,10 +105,10 @@ def WebIDLTest(parser, harness):
           [SecureContext]
           readonly attribute byte testSecureAttribute;
           readonly attribute byte testNonSecureAttribute2;
-          void testNonSecureMethod1(byte foo);
+          undefined testNonSecureMethod1(byte foo);
           [SecureContext]
-          void testSecureMethod(byte foo);
-          void testNonSecureMethod2(byte foo);
+          undefined testSecureMethod(byte foo);
+          undefined testNonSecureMethod2(byte foo);
         };
     """)
     results = parser.finish()
@@ -147,10 +147,10 @@ def WebIDLTest(parser, harness):
           [SecureContext]
           readonly attribute byte testSecureAttribute;
           readonly attribute byte testNonSecureAttribute2;
-          void testNonSecureMethod1(byte foo);
+          undefined testNonSecureMethod1(byte foo);
           [SecureContext]
-          void testSecureMethod(byte foo);
-          void testNonSecureMethod2(byte foo);
+          undefined testSecureMethod(byte foo);
+          undefined testNonSecureMethod2(byte foo);
         };
     """)
     results = parser.finish()
@@ -194,10 +194,10 @@ def WebIDLTest(parser, harness):
         parser.parse("""
             interface TestSecureContextForOverloads1 {
               [SecureContext]
-              void testSecureMethod(byte foo);
+              undefined testSecureMethod(byte foo);
             };
             partial interface TestSecureContextForOverloads1 {
-              void testSecureMethod(byte foo, byte bar);
+              undefined testSecureMethod(byte foo, byte bar);
             };
         """)
         results = parser.finish()
@@ -211,11 +211,11 @@ def WebIDLTest(parser, harness):
         parser.parse("""
             interface TestSecureContextForOverloads2 {
               [SecureContext]
-              void testSecureMethod(byte foo);
+              undefined testSecureMethod(byte foo);
             };
             partial interface TestSecureContextForOverloads2 {
               [SecureContext]
-              void testSecureMethod(byte foo, byte bar);
+              undefined testSecureMethod(byte foo, byte bar);
             };
         """)
         results = parser.finish()
@@ -230,7 +230,7 @@ def WebIDLTest(parser, harness):
             [SecureContext]
             interface TestSecureContextOnInterfaceAndMember {
               [SecureContext]
-              void testSecureMethod(byte foo);
+              undefined testSecureMethod(byte foo);
             };
         """)
         results = parser.finish()
@@ -247,7 +247,7 @@ def WebIDLTest(parser, harness):
             [SecureContext]
             partial interface TestSecureContextOnPartialInterfaceAndMember {
               [SecureContext]
-              void testSecureMethod(byte foo);
+              undefined testSecureMethod(byte foo);
             };
         """)
         results = parser.finish()
@@ -264,7 +264,7 @@ def WebIDLTest(parser, harness):
             };
             partial interface TestSecureContextOnInterfaceAndPartialInterfaceMember {
               [SecureContext]
-              void testSecureMethod(byte foo);
+              undefined testSecureMethod(byte foo);
             };
         """)
         results = parser.finish()
@@ -280,7 +280,7 @@ def WebIDLTest(parser, harness):
             interface TestSecureContextOnInheritedInterface {
             };
             interface TestSecureContextNotOnInheritingInterface : TestSecureContextOnInheritedInterface {
-              void testSecureMethod(byte foo);
+              undefined testSecureMethod(byte foo);
             };
         """)
         results = parser.finish()
@@ -298,7 +298,7 @@ def WebIDLTest(parser, harness):
         interface mixin TestNonSecureContextMixin {
           const octet TEST_CONSTANT_2 = 0;
           readonly attribute byte testAttribute2;
-          void testMethod2(byte foo);
+          undefined testMethod2(byte foo);
         };
         TestSecureContextInterfaceThatIncludesNonSecureContextMixin includes TestNonSecureContextMixin;
      """)
@@ -314,13 +314,13 @@ def WebIDLTest(parser, harness):
                "Attributes copied from non-[SecureContext] mixin should not be [SecureContext]")
     harness.ok(results[0].members[3].getExtendedAttribute("SecureContext") is None,
                "Methods copied from non-[SecureContext] mixin should not be [SecureContext]")
- 
+
     # Test SecureContext and NoInterfaceObject
     parser = parser.reset()
     parser.parse("""
         [NoInterfaceObject, SecureContext]
         interface TestSecureContextNoInterfaceObject {
-          void testSecureMethod(byte foo);
+          undefined testSecureMethod(byte foo);
         };
     """)
     results = parser.finish()
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_special_method_signature_mismatch.py b/components/script/dom/bindings/codegen/parser/tests/test_special_method_signature_mismatch.py
index 52cfcb96817..b209c850d6b 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_special_method_signature_mismatch.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_special_method_signature_mismatch.py
@@ -17,7 +17,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface SpecialMethodSignatureMismatch2 {
-              getter void foo(unsigned long index);
+              getter undefined foo(unsigned long index);
             };
         """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_special_methods.py b/components/script/dom/bindings/codegen/parser/tests/test_special_methods.py
index 7f911733b62..c657c9c797d 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_special_methods.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_special_methods.py
@@ -61,7 +61,7 @@ def WebIDLTest(parser, harness):
         parser.parse(
             """
             interface IndexedDeleter {
-              deleter void(unsigned long index);
+              deleter undefined(unsigned long index);
             };
             """)
         parser.finish()
@@ -69,5 +69,5 @@ def WebIDLTest(parser, harness):
         threw = True
 
     harness.ok(threw, "There are no indexed deleters")
-        
-        
+
+
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_typedef.py b/components/script/dom/bindings/codegen/parser/tests/test_typedef.py
index b5fc1c68890..d98088380ba 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_typedef.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_typedef.py
@@ -4,9 +4,9 @@ def WebIDLTest(parser, harness):
       typedef long? mynullablelong;
       interface Foo {
         const mylong X = 5;
-        void foo(optional mynullablelong arg = 7);
-        void bar(optional mynullablelong arg = null);
-        void baz(mylong arg);
+        undefined foo(optional mynullablelong arg = 7);
+        undefined bar(optional mynullablelong arg = null);
+        undefined baz(mylong arg);
       };
     """)
 
@@ -21,7 +21,7 @@ def WebIDLTest(parser, harness):
         parser.parse("""
           typedef long? mynullablelong;
           interface Foo {
-            void foo(mynullablelong? Y);
+            undefined foo(mynullablelong? Y);
           };
         """)
         results = parser.finish()
@@ -44,7 +44,7 @@ def WebIDLTest(parser, harness):
         threw = True
 
     harness.ok(threw, "Should have thrown on nullable inside nullable const.")
-    
+
     parser = parser.reset()
     threw = False
     try:
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_unforgeable.py b/components/script/dom/bindings/codegen/parser/tests/test_unforgeable.py
index 770a9d3736f..e72548f637f 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_unforgeable.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_unforgeable.py
@@ -47,7 +47,7 @@ def WebIDLTest(parser, harness):
     parser = parser.reset();
     parser.parse("""
             interface Child : Parent {
-              static void foo();
+              static undefined foo();
             };
             interface Parent {
               [Unforgeable] readonly attribute long foo;
@@ -65,7 +65,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface Child : Parent {
-              void foo();
+              undefined foo();
             };
             interface Parent {
               [Unforgeable] readonly attribute long foo;
@@ -84,10 +84,10 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface Child : Parent {
-              void foo();
+              undefined foo();
             };
             interface Parent {
-              [Unforgeable] void foo();
+              [Unforgeable] undefined foo();
             };
         """)
 
@@ -125,7 +125,7 @@ def WebIDLTest(parser, harness):
               attribute short foo;
             };
             interface Parent {
-              [Unforgeable] void foo();
+              [Unforgeable] undefined foo();
             };
         """)
 
@@ -157,7 +157,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface Child : Parent {
-              void foo();
+              undefined foo();
             };
             interface Parent {};
             interface mixin Mixin {
@@ -187,7 +187,7 @@ def WebIDLTest(parser, harness):
             };
             GrandParent includes Mixin;
             interface mixin ChildMixin {
-              void foo();
+              undefined foo();
             };
             Child includes ChildMixin;
         """)
@@ -209,11 +209,11 @@ def WebIDLTest(parser, harness):
             interface Parent : GrandParent {};
             interface GrandParent {};
             interface mixin Mixin {
-              [Unforgeable] void foo();
+              [Unforgeable] undefined foo();
             };
             GrandParent includes Mixin;
             interface mixin ChildMixin {
-              void foo();
+              undefined foo();
             };
             Child includes ChildMixin;
         """)
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_union.py b/components/script/dom/bindings/codegen/parser/tests/test_union.py
index 801314fd0bd..469208b264d 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_union.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_union.py
@@ -136,10 +136,10 @@ def WebIDLTest(parser, harness):
         """
     for (i, type) in enumerate(validUnionTypes):
         interface += string.Template("""
-          void method${i}(${type} arg);
+          undefined method${i}(${type} arg);
           ${type} returnMethod${i}();
           attribute ${type} attr${i};
-          void optionalMethod${i}(${type}? arg);
+          undefined optionalMethod${i}(${type}? arg);
         """).substitute(i=i, type=type)
     interface += """
         };
@@ -152,7 +152,7 @@ def WebIDLTest(parser, harness):
     for invalid in invalidUnionTypes:
         interface = testPre + string.Template("""
             interface TestUnion {
-              void method(${type} arg);
+              undefined method(${type} arg);
             };
         """).substitute(type=invalid)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_union_any.py b/components/script/dom/bindings/codegen/parser/tests/test_union_any.py
index e34cadab470..3eb67648d56 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_union_any.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_union_any.py
@@ -3,7 +3,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface AnyNotInUnion {
-              void foo((any or DOMString) arg);
+              undefined foo((any or DOMString) arg);
             };
         """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_union_nullable.py b/components/script/dom/bindings/codegen/parser/tests/test_union_nullable.py
index 08430a94a2e..71da4349e6e 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_union_nullable.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_union_nullable.py
@@ -3,7 +3,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface OneNullableInUnion {
-              void foo((object? or DOMString?) arg);
+              undefined foo((object? or DOMString?) arg);
             };
         """)
 
@@ -20,7 +20,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface NullableInNullableUnion {
-              void foo((object? or DOMString)? arg);
+              undefined foo((object? or DOMString)? arg);
             };
         """)
 
@@ -40,7 +40,7 @@ def WebIDLTest(parser, harness):
             interface NullableInUnionNullableUnionHelper {
             };
             interface NullableInUnionNullableUnion {
-              void foo(((object? or DOMString) or NullableInUnionNullableUnionHelper)? arg);
+              undefined foo(((object? or DOMString) or NullableInUnionNullableUnionHelper)? arg);
             };
         """)
 
diff --git a/components/script/dom/bindings/codegen/parser/tests/test_variadic_constraints.py b/components/script/dom/bindings/codegen/parser/tests/test_variadic_constraints.py
index 7448e40d5a9..e36eff8b476 100644
--- a/components/script/dom/bindings/codegen/parser/tests/test_variadic_constraints.py
+++ b/components/script/dom/bindings/codegen/parser/tests/test_variadic_constraints.py
@@ -3,7 +3,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface VariadicConstraints1 {
-              void foo(byte... arg1, byte arg2);
+              undefined foo(byte... arg1, byte arg2);
             };
         """)
         results = parser.finish()
@@ -20,7 +20,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface VariadicConstraints2 {
-              void foo(byte... arg1, optional byte arg2);
+              undefined foo(byte... arg1, optional byte arg2);
             };
         """)
         results = parser.finish();
@@ -36,7 +36,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface VariadicConstraints3 {
-              void foo(optional byte... arg1);
+              undefined foo(optional byte... arg1);
             };
         """)
         results = parser.finish()
@@ -53,7 +53,7 @@ def WebIDLTest(parser, harness):
     try:
         parser.parse("""
             interface VariadicConstraints4 {
-              void foo(byte... arg1 = 0);
+              undefined foo(byte... arg1 = 0);
             };
         """)
         results = parser.finish()
diff --git a/components/script/dom/webidls/ANGLEInstancedArrays.webidl b/components/script/dom/webidls/ANGLEInstancedArrays.webidl
index fc7e5d3efab..4ec6cdffc4e 100644
--- a/components/script/dom/webidls/ANGLEInstancedArrays.webidl
+++ b/components/script/dom/webidls/ANGLEInstancedArrays.webidl
@@ -9,7 +9,7 @@
 [NoInterfaceObject, Exposed=Window]
 interface ANGLEInstancedArrays {
     const GLenum VERTEX_ATTRIB_ARRAY_DIVISOR_ANGLE = 0x88FE;
-    void drawArraysInstancedANGLE(GLenum mode, GLint first, GLsizei count, GLsizei primcount);
-    void drawElementsInstancedANGLE(GLenum mode, GLsizei count, GLenum type, GLintptr offset, GLsizei primcount);
-    void vertexAttribDivisorANGLE(GLuint index, GLuint divisor);
+    undefined drawArraysInstancedANGLE(GLenum mode, GLint first, GLsizei count, GLsizei primcount);
+    undefined drawElementsInstancedANGLE(GLenum mode, GLsizei count, GLenum type, GLintptr offset, GLsizei primcount);
+    undefined vertexAttribDivisorANGLE(GLuint index, GLuint divisor);
 };
diff --git a/components/script/dom/webidls/ActivatableElement.webidl b/components/script/dom/webidls/ActivatableElement.webidl
index bce7730a833..e9d959c8470 100644
--- a/components/script/dom/webidls/ActivatableElement.webidl
+++ b/components/script/dom/webidls/ActivatableElement.webidl
@@ -8,8 +8,8 @@
 [Exposed=(Window,Worker)]
 interface mixin ActivatableElement {
   [Throws, Pref="dom.testing.element.activation.enabled"]
-  void enterFormalActivationState();
+  undefined enterFormalActivationState();
 
   [Throws, Pref="dom.testing.element.activation.enabled"]
-  void exitFormalActivationState();
+  undefined exitFormalActivationState();
 };
diff --git a/components/script/dom/webidls/AnalyserNode.webidl b/components/script/dom/webidls/AnalyserNode.webidl
index c1398d56378..5bde7c7eb5e 100644
--- a/components/script/dom/webidls/AnalyserNode.webidl
+++ b/components/script/dom/webidls/AnalyserNode.webidl
@@ -16,10 +16,10 @@ dictionary AnalyserOptions : AudioNodeOptions {
 [Exposed=Window]
 interface AnalyserNode : AudioNode {
   [Throws] constructor(BaseAudioContext context, optional AnalyserOptions options = {});
-  void getFloatFrequencyData (Float32Array array);
-  void getByteFrequencyData (Uint8Array array);
-  void getFloatTimeDomainData (Float32Array array);
-  void getByteTimeDomainData (Uint8Array array);
+  undefined getFloatFrequencyData (Float32Array array);
+  undefined getByteFrequencyData (Uint8Array array);
+  undefined getFloatTimeDomainData (Float32Array array);
+  undefined getByteTimeDomainData (Uint8Array array);
   [SetterThrows] attribute unsigned long fftSize;
   readonly attribute unsigned long frequencyBinCount;
   [SetterThrows] attribute double minDecibels;
diff --git a/components/script/dom/webidls/AudioBuffer.webidl b/components/script/dom/webidls/AudioBuffer.webidl
index 75b3b470838..56a735ec3f3 100644
--- a/components/script/dom/webidls/AudioBuffer.webidl
+++ b/components/script/dom/webidls/AudioBuffer.webidl
@@ -20,10 +20,10 @@ interface AudioBuffer {
   readonly attribute double duration;
   readonly attribute unsigned long numberOfChannels;
   [Throws] Float32Array getChannelData(unsigned long channel);
-  [Throws] void copyFromChannel(Float32Array destination,
+  [Throws] undefined copyFromChannel(Float32Array destination,
                                 unsigned long channelNumber,
                                 optional unsigned long startInChannel = 0);
-  [Throws] void copyToChannel(Float32Array source,
+  [Throws] undefined copyToChannel(Float32Array source,
                               unsigned long channelNumber,
                               optional unsigned long startInChannel = 0);
 };
diff --git a/components/script/dom/webidls/AudioBufferSourceNode.webidl b/components/script/dom/webidls/AudioBufferSourceNode.webidl
index 8744a521ddb..7c0d3538b1d 100644
--- a/components/script/dom/webidls/AudioBufferSourceNode.webidl
+++ b/components/script/dom/webidls/AudioBufferSourceNode.webidl
@@ -24,7 +24,7 @@ interface AudioBufferSourceNode : AudioScheduledSourceNode {
   attribute boolean loop;
   attribute double loopStart;
   attribute double loopEnd;
-  [Throws] void start(optional double when = 0,
+  [Throws] undefined start(optional double when = 0,
                       optional double offset,
                       optional double duration);
 };
diff --git a/components/script/dom/webidls/AudioContext.webidl b/components/script/dom/webidls/AudioContext.webidl
index 09d6693b690..8f36c54d0fc 100644
--- a/components/script/dom/webidls/AudioContext.webidl
+++ b/components/script/dom/webidls/AudioContext.webidl
@@ -30,8 +30,8 @@ interface AudioContext : BaseAudioContext {
 
   AudioTimestamp getOutputTimestamp();
 
-  Promise<void> suspend();
-  Promise<void> close();
+  Promise<undefined> suspend();
+  Promise<undefined> close();
 
   [Throws] MediaElementAudioSourceNode createMediaElementSource(HTMLMediaElement mediaElement);
   [Throws] MediaStreamAudioSourceNode createMediaStreamSource(MediaStream mediaStream);
diff --git a/components/script/dom/webidls/AudioNode.webidl b/components/script/dom/webidls/AudioNode.webidl
index bf4f88e02b6..45d97bc5762 100644
--- a/components/script/dom/webidls/AudioNode.webidl
+++ b/components/script/dom/webidls/AudioNode.webidl
@@ -30,24 +30,24 @@ interface AudioNode : EventTarget {
                     optional unsigned long output = 0,
                     optional unsigned long input = 0);
   [Throws]
-  void connect(AudioParam destinationParam,
+  undefined connect(AudioParam destinationParam,
                optional unsigned long output = 0);
   [Throws]
-  void disconnect();
+  undefined disconnect();
   [Throws]
-  void disconnect(unsigned long output);
+  undefined disconnect(unsigned long output);
   [Throws]
-  void disconnect(AudioNode destination);
+  undefined disconnect(AudioNode destination);
   [Throws]
-  void disconnect(AudioNode destination, unsigned long output);
+  undefined disconnect(AudioNode destination, unsigned long output);
   [Throws]
-  void disconnect(AudioNode destination,
+  undefined disconnect(AudioNode destination,
                   unsigned long output,
                   unsigned long input);
   [Throws]
-  void disconnect(AudioParam destination);
+  undefined disconnect(AudioParam destination);
   [Throws]
-  void disconnect(AudioParam destination, unsigned long output);
+  undefined disconnect(AudioParam destination, unsigned long output);
 
   readonly attribute BaseAudioContext context;
   readonly attribute unsigned long numberOfInputs;
diff --git a/components/script/dom/webidls/AudioScheduledSourceNode.webidl b/components/script/dom/webidls/AudioScheduledSourceNode.webidl
index 6be6373a7ec..32542cbfa51 100644
--- a/components/script/dom/webidls/AudioScheduledSourceNode.webidl
+++ b/components/script/dom/webidls/AudioScheduledSourceNode.webidl
@@ -9,6 +9,6 @@
 [Exposed=Window]
 interface AudioScheduledSourceNode : AudioNode {
   attribute EventHandler onended;
-  [Throws] void start(optional double when = 0);
-  [Throws] void stop(optional double when = 0);
+  [Throws] undefined start(optional double when = 0);
+  [Throws] undefined stop(optional double when = 0);
 };
diff --git a/components/script/dom/webidls/BaseAudioContext.webidl b/components/script/dom/webidls/BaseAudioContext.webidl
index 57fb677defd..46a7bb945b7 100644
--- a/components/script/dom/webidls/BaseAudioContext.webidl
+++ b/components/script/dom/webidls/BaseAudioContext.webidl
@@ -12,8 +12,8 @@ enum AudioContextState {
   "closed"
 };
 
-callback DecodeErrorCallback = void (DOMException error);
-callback DecodeSuccessCallback = void (AudioBuffer decodedData);
+callback DecodeErrorCallback = undefined (DOMException error);
+callback DecodeSuccessCallback = undefined (AudioBuffer decodedData);
 
 [Exposed=Window]
 interface BaseAudioContext : EventTarget {
@@ -22,7 +22,7 @@ interface BaseAudioContext : EventTarget {
   readonly attribute double currentTime;
   readonly attribute AudioListener listener;
   readonly attribute AudioContextState  state;
-  Promise<void> resume();
+  Promise<undefined> resume();
   attribute EventHandler onstatechange;
   [Throws] AudioBuffer createBuffer(unsigned long numberOfChannels,
                                     unsigned long length,
diff --git a/components/script/dom/webidls/BluetoothDevice.webidl b/components/script/dom/webidls/BluetoothDevice.webidl
index 8ead2168146..c514132140d 100644
--- a/components/script/dom/webidls/BluetoothDevice.webidl
+++ b/components/script/dom/webidls/BluetoothDevice.webidl
@@ -10,8 +10,8 @@ interface BluetoothDevice : EventTarget {
   readonly attribute DOMString? name;
   readonly attribute BluetoothRemoteGATTServer? gatt;
 
-  Promise<void> watchAdvertisements();
-  void unwatchAdvertisements();
+  Promise<undefined> watchAdvertisements();
+  undefined unwatchAdvertisements();
   readonly attribute boolean watchingAdvertisements;
 };
 
diff --git a/components/script/dom/webidls/BluetoothRemoteGATTCharacteristic.webidl b/components/script/dom/webidls/BluetoothRemoteGATTCharacteristic.webidl
index af85d1c1860..9f41d2450f9 100644
--- a/components/script/dom/webidls/BluetoothRemoteGATTCharacteristic.webidl
+++ b/components/script/dom/webidls/BluetoothRemoteGATTCharacteristic.webidl
@@ -16,7 +16,7 @@ interface BluetoothRemoteGATTCharacteristic : EventTarget {
   getDescriptors(optional BluetoothDescriptorUUID descriptor);
   Promise<ByteString> readValue();
   //Promise<DataView> readValue();
-  Promise<void> writeValue(BufferSource value);
+  Promise<undefined> writeValue(BufferSource value);
   Promise<BluetoothRemoteGATTCharacteristic> startNotifications();
   Promise<BluetoothRemoteGATTCharacteristic> stopNotifications();
 };
diff --git a/components/script/dom/webidls/BluetoothRemoteGATTDescriptor.webidl b/components/script/dom/webidls/BluetoothRemoteGATTDescriptor.webidl
index 37c8722e224..29cbc3040a4 100644
--- a/components/script/dom/webidls/BluetoothRemoteGATTDescriptor.webidl
+++ b/components/script/dom/webidls/BluetoothRemoteGATTDescriptor.webidl
@@ -12,5 +12,5 @@ interface BluetoothRemoteGATTDescriptor {
   readonly attribute ByteString? value;
   Promise<ByteString> readValue();
   //Promise<DataView> readValue();
-  Promise<void> writeValue(BufferSource value);
+  Promise<undefined> writeValue(BufferSource value);
 };
diff --git a/components/script/dom/webidls/BluetoothRemoteGATTServer.webidl b/components/script/dom/webidls/BluetoothRemoteGATTServer.webidl
index 324750dc39b..e46cef4a079 100644
--- a/components/script/dom/webidls/BluetoothRemoteGATTServer.webidl
+++ b/components/script/dom/webidls/BluetoothRemoteGATTServer.webidl
@@ -11,7 +11,7 @@ interface BluetoothRemoteGATTServer {
   readonly attribute boolean connected;
   Promise<BluetoothRemoteGATTServer> connect();
   [Throws]
-  void disconnect();
+  undefined disconnect();
   Promise<BluetoothRemoteGATTService> getPrimaryService(BluetoothServiceUUID service);
   Promise<sequence<BluetoothRemoteGATTService>> getPrimaryServices(optional BluetoothServiceUUID service);
 };
diff --git a/components/script/dom/webidls/BroadcastChannel.webidl b/components/script/dom/webidls/BroadcastChannel.webidl
index 6d72f3997cf..886ea5761c6 100644
--- a/components/script/dom/webidls/BroadcastChannel.webidl
+++ b/components/script/dom/webidls/BroadcastChannel.webidl
@@ -11,8 +11,8 @@ interface BroadcastChannel : EventTarget {
   constructor(DOMString name);
 
   readonly attribute DOMString name;
-  [Throws] void postMessage(any message);
-  void close();
+  [Throws] undefined postMessage(any message);
+  undefined close();
   attribute EventHandler onmessage;
   attribute EventHandler onmessageerror;
 };
diff --git a/components/script/dom/webidls/CSSGroupingRule.webidl b/components/script/dom/webidls/CSSGroupingRule.webidl
index 3e21e85ecc4..7707f38d984 100644
--- a/components/script/dom/webidls/CSSGroupingRule.webidl
+++ b/components/script/dom/webidls/CSSGroupingRule.webidl
@@ -7,6 +7,6 @@
 interface CSSGroupingRule : CSSRule {
   [SameObject] readonly attribute CSSRuleList cssRules;
   [Throws] unsigned long insertRule(DOMString rule, unsigned long index);
-  [Throws] void deleteRule(unsigned long index);
+  [Throws] undefined deleteRule(unsigned long index);
 };
 
diff --git a/components/script/dom/webidls/CSSKeyframesRule.webidl b/components/script/dom/webidls/CSSKeyframesRule.webidl
index e1f30b19015..b812d124733 100644
--- a/components/script/dom/webidls/CSSKeyframesRule.webidl
+++ b/components/script/dom/webidls/CSSKeyframesRule.webidl
@@ -9,7 +9,7 @@ interface CSSKeyframesRule : CSSRule {
            attribute DOMString   name;
   readonly attribute CSSRuleList cssRules;
 
-  void            appendRule(DOMString rule);
-  void            deleteRule(DOMString select);
+  undefined            appendRule(DOMString rule);
+  undefined            deleteRule(DOMString select);
   CSSKeyframeRule? findRule(DOMString select);
 };
diff --git a/components/script/dom/webidls/CSSStyleDeclaration.webidl b/components/script/dom/webidls/CSSStyleDeclaration.webidl
index 7865195360f..1b67ec6a7fa 100644
--- a/components/script/dom/webidls/CSSStyleDeclaration.webidl
+++ b/components/script/dom/webidls/CSSStyleDeclaration.webidl
@@ -17,7 +17,7 @@ interface CSSStyleDeclaration {
   DOMString getPropertyValue(DOMString property);
   DOMString getPropertyPriority(DOMString property);
   [CEReactions, Throws]
-  void setProperty(DOMString property, [TreatNullAs=EmptyString] DOMString value,
+  undefined setProperty(DOMString property, [TreatNullAs=EmptyString] DOMString value,
                                        optional [TreatNullAs=EmptyString] DOMString priority = "");
   [CEReactions, Throws]
   DOMString removeProperty(DOMString property);
diff --git a/components/script/dom/webidls/CSSStyleSheet.webidl b/components/script/dom/webidls/CSSStyleSheet.webidl
index c09f14a3e8a..0133eefd8a1 100644
--- a/components/script/dom/webidls/CSSStyleSheet.webidl
+++ b/components/script/dom/webidls/CSSStyleSheet.webidl
@@ -8,5 +8,5 @@ interface CSSStyleSheet : StyleSheet {
   // readonly attribute CSSRule? ownerRule;
   [Throws, SameObject] readonly attribute CSSRuleList cssRules;
   [Throws] unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
-  [Throws] void deleteRule(unsigned long index);
+  [Throws] undefined deleteRule(unsigned long index);
 };
diff --git a/components/script/dom/webidls/CanvasGradient.webidl b/components/script/dom/webidls/CanvasGradient.webidl
index ec00d4a73f2..46578087beb 100644
--- a/components/script/dom/webidls/CanvasGradient.webidl
+++ b/components/script/dom/webidls/CanvasGradient.webidl
@@ -7,7 +7,7 @@
 interface CanvasGradient {
   // opaque object
   [Throws]
-  void addColorStop(double offset, DOMString color);
+  undefined addColorStop(double offset, DOMString color);
 };
 
 
diff --git a/components/script/dom/webidls/CanvasRenderingContext2D.webidl b/components/script/dom/webidls/CanvasRenderingContext2D.webidl
index 6216ec50505..2c2cc0a8e5d 100644
--- a/components/script/dom/webidls/CanvasRenderingContext2D.webidl
+++ b/components/script/dom/webidls/CanvasRenderingContext2D.webidl
@@ -42,17 +42,17 @@ CanvasRenderingContext2D includes CanvasPath;
 [Exposed=(PaintWorklet, Window, Worker)]
 interface mixin CanvasState {
   // state
-  void save(); // push state on state stack
-  void restore(); // pop state stack and restore state
+  undefined save(); // push state on state stack
+  undefined restore(); // pop state stack and restore state
 };
 
 [Exposed=(PaintWorklet, Window, Worker)]
 interface mixin CanvasTransform {
   // transformations (default transform is the identity matrix)
-  void scale(unrestricted double x, unrestricted double y);
-  void rotate(unrestricted double angle);
-  void translate(unrestricted double x, unrestricted double y);
-  void transform(unrestricted double a,
+  undefined scale(unrestricted double x, unrestricted double y);
+  undefined rotate(unrestricted double angle);
+  undefined translate(unrestricted double x, unrestricted double y);
+  undefined transform(unrestricted double a,
                  unrestricted double b,
                  unrestricted double c,
                  unrestricted double d,
@@ -60,14 +60,14 @@ interface mixin CanvasTransform {
                  unrestricted double f);
 
   [NewObject] DOMMatrix getTransform();
-  void setTransform(unrestricted double a,
+  undefined setTransform(unrestricted double a,
                     unrestricted double b,
                     unrestricted double c,
                     unrestricted double d,
                     unrestricted double e,
                     unrestricted double f);
   // void setTransform(optional DOMMatrixInit matrix);
-  void resetTransform();
+  undefined resetTransform();
 };
 
 [Exposed=(PaintWorklet, Window, Worker)]
@@ -114,20 +114,20 @@ interface mixin CanvasFilters {
 [Exposed=(PaintWorklet, Window, Worker)]
 interface mixin CanvasRect {
   // rects
-  void clearRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
-  void fillRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
-  void strokeRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+  undefined clearRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+  undefined fillRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+  undefined strokeRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
 };
 
 [Exposed=(PaintWorklet, Window, Worker)]
 interface mixin CanvasDrawPath {
   // path API (see also CanvasPath)
-  void beginPath();
-  void fill(optional CanvasFillRule fillRule = "nonzero");
+  undefined beginPath();
+  undefined fill(optional CanvasFillRule fillRule = "nonzero");
   //void fill(Path2D path, optional CanvasFillRule fillRule = "nonzero");
-  void stroke();
+  undefined stroke();
   //void stroke(Path2D path);
-  void clip(optional CanvasFillRule fillRule = "nonzero");
+  undefined clip(optional CanvasFillRule fillRule = "nonzero");
   //void clip(Path2D path, optional CanvasFillRule fillRule = "nonzero");
   boolean isPointInPath(unrestricted double x, unrestricted double y,
                         optional CanvasFillRule fillRule = "nonzero");
@@ -149,7 +149,7 @@ interface mixin CanvasUserInterface {
 interface mixin CanvasText {
   // text (see also the CanvasPathDrawingStyles and CanvasTextDrawingStyles interfaces)
   [Pref="dom.canvas_text.enabled"]
-  void fillText(DOMString text, unrestricted double x, unrestricted double y,
+  undefined fillText(DOMString text, unrestricted double x, unrestricted double y,
                 optional unrestricted double maxWidth);
   //void strokeText(DOMString text, unrestricted double x, unrestricted double y,
   //                optional unrestricted double maxWidth);
@@ -161,12 +161,12 @@ interface mixin CanvasText {
 interface mixin CanvasDrawImage {
   // drawing images
   [Throws]
-  void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy);
+  undefined drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy);
   [Throws]
-  void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy,
+  undefined drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy,
                                           unrestricted double dw, unrestricted double dh);
   [Throws]
-  void drawImage(CanvasImageSource image, unrestricted double sx, unrestricted double sy,
+  undefined drawImage(CanvasImageSource image, unrestricted double sx, unrestricted double sy,
                                           unrestricted double sw, unrestricted double sh,
                                           unrestricted double dx, unrestricted double dy,
                                           unrestricted double dw, unrestricted double dh);
@@ -181,8 +181,8 @@ interface mixin CanvasImageData {
   ImageData createImageData(ImageData imagedata);
   [Throws]
   ImageData getImageData(long sx, long sy, long sw, long sh);
-  void putImageData(ImageData imagedata, long dx, long dy);
-  void putImageData(ImageData imagedata,
+  undefined putImageData(ImageData imagedata, long dx, long dy);
+  undefined putImageData(ImageData imagedata,
                     long dx, long dy,
                     long dirtyX, long dirtyY,
                     long dirtyWidth, long dirtyHeight);
@@ -221,13 +221,13 @@ interface mixin CanvasTextDrawingStyles {
 [Exposed=(PaintWorklet, Window, Worker)]
 interface mixin CanvasPath {
   // shared path API methods
-  void closePath();
-  void moveTo(unrestricted double x, unrestricted double y);
-  void lineTo(unrestricted double x, unrestricted double y);
-  void quadraticCurveTo(unrestricted double cpx, unrestricted double cpy,
+  undefined closePath();
+  undefined moveTo(unrestricted double x, unrestricted double y);
+  undefined lineTo(unrestricted double x, unrestricted double y);
+  undefined quadraticCurveTo(unrestricted double cpx, unrestricted double cpy,
                         unrestricted double x, unrestricted double y);
 
-  void bezierCurveTo(unrestricted double cp1x,
+  undefined bezierCurveTo(unrestricted double cp1x,
                      unrestricted double cp1y,
                      unrestricted double cp2x,
                      unrestricted double cp2y,
@@ -235,18 +235,18 @@ interface mixin CanvasPath {
                      unrestricted double y);
 
   [Throws]
-  void arcTo(unrestricted double x1, unrestricted double y1,
+  undefined arcTo(unrestricted double x1, unrestricted double y1,
              unrestricted double x2, unrestricted double y2,
              unrestricted double radius);
 
-  void rect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+  undefined rect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
 
   [Throws]
-  void arc(unrestricted double x, unrestricted double y, unrestricted double radius,
+  undefined arc(unrestricted double x, unrestricted double y, unrestricted double radius,
            unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false);
 
   [Throws]
-  void ellipse(unrestricted double x, unrestricted double y, unrestricted double radius_x,
+  undefined ellipse(unrestricted double x, unrestricted double y, unrestricted double radius_x,
                unrestricted double radius_y, unrestricted double rotation, unrestricted double startAngle,
                unrestricted double endAngle, optional boolean anticlockwise = false);
 };
diff --git a/components/script/dom/webidls/CharacterData.webidl b/components/script/dom/webidls/CharacterData.webidl
index e69f862afc9..34dc42c2db8 100644
--- a/components/script/dom/webidls/CharacterData.webidl
+++ b/components/script/dom/webidls/CharacterData.webidl
@@ -15,13 +15,13 @@ interface CharacterData : Node {
   [Pure] readonly attribute unsigned long length;
   [Pure, Throws]
   DOMString substringData(unsigned long offset, unsigned long count);
-  void appendData(DOMString data);
+  undefined appendData(DOMString data);
   [Throws]
-  void insertData(unsigned long offset, DOMString data);
+  undefined insertData(unsigned long offset, DOMString data);
   [Throws]
-  void deleteData(unsigned long offset, unsigned long count);
+  undefined deleteData(unsigned long offset, unsigned long count);
   [Throws]
-  void replaceData(unsigned long offset, unsigned long count, DOMString data);
+  undefined replaceData(unsigned long offset, unsigned long count, DOMString data);
 };
 
 CharacterData includes ChildNode;
diff --git a/components/script/dom/webidls/ChildNode.webidl b/components/script/dom/webidls/ChildNode.webidl
index 8964653fffc..ec8ad17aa2f 100644
--- a/components/script/dom/webidls/ChildNode.webidl
+++ b/components/script/dom/webidls/ChildNode.webidl
@@ -8,13 +8,13 @@
 
 interface mixin ChildNode {
   [Throws, CEReactions, Unscopable]
-  void before((Node or DOMString)... nodes);
+  undefined before((Node or DOMString)... nodes);
   [Throws, CEReactions, Unscopable]
-  void after((Node or DOMString)... nodes);
+  undefined after((Node or DOMString)... nodes);
   [Throws, CEReactions, Unscopable]
-  void replaceWith((Node or DOMString)... nodes);
+  undefined replaceWith((Node or DOMString)... nodes);
   [CEReactions, Unscopable]
-  void remove();
+  undefined remove();
 };
 
 interface mixin NonDocumentTypeChildNode {
diff --git a/components/script/dom/webidls/Console.webidl b/components/script/dom/webidls/Console.webidl
index c41e46d8535..19cff567d6a 100644
--- a/components/script/dom/webidls/Console.webidl
+++ b/components/script/dom/webidls/Console.webidl
@@ -9,20 +9,20 @@
  ProtoObjectHack]
 namespace console {
   // Logging
-  void log(DOMString... messages);
-  void debug(DOMString... messages);
-  void info(DOMString... messages);
-  void warn(DOMString... messages);
-  void error(DOMString... messages);
-  void assert(boolean condition, optional DOMString message);
-  void clear();
+  undefined log(DOMString... messages);
+  undefined debug(DOMString... messages);
+  undefined info(DOMString... messages);
+  undefined warn(DOMString... messages);
+  undefined error(DOMString... messages);
+  undefined assert(boolean condition, optional DOMString message);
+  undefined clear();
 
   // Grouping
-  void group(DOMString... data);
-  void groupCollapsed(DOMString... data);
-  void groupEnd();
+  undefined group(DOMString... data);
+  undefined groupCollapsed(DOMString... data);
+  undefined groupEnd();
 
   // Timing
-  void time(DOMString message);
-  void timeEnd(DOMString message);
+  undefined time(DOMString message);
+  undefined timeEnd(DOMString message);
 };
diff --git a/components/script/dom/webidls/CustomElementRegistry.webidl b/components/script/dom/webidls/CustomElementRegistry.webidl
index 1d6f638d67e..8f9347296a9 100644
--- a/components/script/dom/webidls/CustomElementRegistry.webidl
+++ b/components/script/dom/webidls/CustomElementRegistry.webidl
@@ -6,13 +6,17 @@
 [Exposed=Window, Pref="dom.custom_elements.enabled"]
 interface CustomElementRegistry {
   [Throws, CEReactions]
-  void define(DOMString name, CustomElementConstructor constructor_, optional ElementDefinitionOptions options = {});
+  undefined define(
+    DOMString name,
+    CustomElementConstructor constructor_,
+    optional ElementDefinitionOptions options = {}
+  );
 
   any get(DOMString name);
 
   Promise<CustomElementConstructor> whenDefined(DOMString name);
 
-  [CEReactions] void upgrade(Node root);
+  [CEReactions] undefined upgrade(Node root);
 };
 
 callback CustomElementConstructor = HTMLElement();
diff --git a/components/script/dom/webidls/CustomEvent.webidl b/components/script/dom/webidls/CustomEvent.webidl
index 604286881df..36e01e0d9b2 100644
--- a/components/script/dom/webidls/CustomEvent.webidl
+++ b/components/script/dom/webidls/CustomEvent.webidl
@@ -18,7 +18,7 @@ interface CustomEvent : Event {
   [Throws] constructor(DOMString type, optional CustomEventInit eventInitDict = {});
   readonly attribute any detail;
 
-  void initCustomEvent(DOMString type, boolean bubbles, boolean cancelable, any detail);
+  undefined initCustomEvent(DOMString type, boolean bubbles, boolean cancelable, any detail);
 };
 
 dictionary CustomEventInit : EventInit {
diff --git a/components/script/dom/webidls/DOMStringMap.webidl b/components/script/dom/webidls/DOMStringMap.webidl
index 0acf6f70228..39891f24096 100644
--- a/components/script/dom/webidls/DOMStringMap.webidl
+++ b/components/script/dom/webidls/DOMStringMap.webidl
@@ -7,7 +7,7 @@
 interface DOMStringMap {
   getter DOMString (DOMString name);
   [CEReactions, Throws]
-  setter void (DOMString name, DOMString value);
+  setter undefined (DOMString name, DOMString value);
   [CEReactions]
-  deleter void (DOMString name);
+  deleter undefined (DOMString name);
 };
diff --git a/components/script/dom/webidls/DOMTokenList.webidl b/components/script/dom/webidls/DOMTokenList.webidl
index b67dbb1a1dc..6be5eb4f232 100644
--- a/components/script/dom/webidls/DOMTokenList.webidl
+++ b/components/script/dom/webidls/DOMTokenList.webidl
@@ -13,9 +13,9 @@ interface DOMTokenList {
   [Pure]
   boolean contains(DOMString token);
   [CEReactions, Throws]
-  void add(DOMString... tokens);
+  undefined add(DOMString... tokens);
   [CEReactions, Throws]
-  void remove(DOMString... tokens);
+  undefined remove(DOMString... tokens);
   [CEReactions, Throws]
   boolean toggle(DOMString token, optional boolean force);
   [CEReactions, Throws]
diff --git a/components/script/dom/webidls/DedicatedWorkerGlobalScope.webidl b/components/script/dom/webidls/DedicatedWorkerGlobalScope.webidl
index 2df97dbacfe..43ff22b5f6d 100644
--- a/components/script/dom/webidls/DedicatedWorkerGlobalScope.webidl
+++ b/components/script/dom/webidls/DedicatedWorkerGlobalScope.webidl
@@ -5,9 +5,9 @@
 // https://html.spec.whatwg.org/multipage/#dedicatedworkerglobalscope
 [Global=(Worker,DedicatedWorker), Exposed=DedicatedWorker]
 /*sealed*/ interface DedicatedWorkerGlobalScope : WorkerGlobalScope {
-  [Throws] void postMessage(any message, sequence<object> transfer);
-  [Throws] void postMessage(any message, optional PostMessageOptions options = {});
+  [Throws] undefined postMessage(any message, sequence<object> transfer);
+  [Throws] undefined postMessage(any message, optional PostMessageOptions options = {});
   attribute EventHandler onmessage;
 
-  void close();
+  undefined close();
 };
diff --git a/components/script/dom/webidls/DissimilarOriginLocation.webidl b/components/script/dom/webidls/DissimilarOriginLocation.webidl
index 8c077665704..20821e48fc6 100644
--- a/components/script/dom/webidls/DissimilarOriginLocation.webidl
+++ b/components/script/dom/webidls/DissimilarOriginLocation.webidl
@@ -17,9 +17,9 @@
 [Exposed=(Window,DissimilarOriginWindow), Unforgeable, NoInterfaceObject]
 interface DissimilarOriginLocation {
   [Throws] attribute USVString href;
-  [Throws] void assign(USVString url);
-  [Throws] void replace(USVString url);
-  [Throws] void reload();
+  [Throws] undefined assign(USVString url);
+  [Throws] undefined replace(USVString url);
+  [Throws] undefined reload();
   [Throws] stringifier;
 
   // TODO: finish this interface
diff --git a/components/script/dom/webidls/DissimilarOriginWindow.webidl b/components/script/dom/webidls/DissimilarOriginWindow.webidl
index b690736469a..755409c7964 100644
--- a/components/script/dom/webidls/DissimilarOriginWindow.webidl
+++ b/components/script/dom/webidls/DissimilarOriginWindow.webidl
@@ -23,11 +23,11 @@ interface DissimilarOriginWindow : GlobalScope {
   [Replaceable] readonly attribute unsigned long length;
   [Unforgeable] readonly attribute DissimilarOriginLocation location;
 
-  void close();
+  undefined close();
   readonly attribute boolean closed;
-  [Throws] void postMessage(any message, USVString targetOrigin, optional sequence<object> transfer = []);
-  [Throws] void postMessage(any message, optional WindowPostMessageOptions options = {});
+  [Throws] undefined postMessage(any message, USVString targetOrigin, optional sequence<object> transfer = []);
+  [Throws] undefined postMessage(any message, optional WindowPostMessageOptions options = {});
   attribute any opener;
-  void blur();
-  void focus();
+  undefined blur();
+  undefined focus();
 };
diff --git a/components/script/dom/webidls/Document.webidl b/components/script/dom/webidls/Document.webidl
index d2429e51489..3f169a35153 100644
--- a/components/script/dom/webidls/Document.webidl
+++ b/components/script/dom/webidls/Document.webidl
@@ -126,11 +126,11 @@ partial /*sealed*/ interface Document {
   [CEReactions, Throws]
   WindowProxy? open(USVString url, DOMString name, DOMString features);
   [CEReactions, Throws]
-  void close();
+  undefined close();
   [CEReactions, Throws]
-  void write(DOMString... text);
+  undefined write(DOMString... text);
   [CEReactions, Throws]
-  void writeln(DOMString... text);
+  undefined writeln(DOMString... text);
 
   // user interaction
   readonly attribute Window?/*Proxy?*/ defaultView;
@@ -179,9 +179,9 @@ partial interface Document {
   [SameObject]
   readonly attribute HTMLCollection applets;
 
-  void clear();
-  void captureEvents();
-  void releaseEvents();
+  undefined clear();
+  undefined captureEvents();
+  undefined releaseEvents();
 
   // Tracking issue for document.all: https://github.com/servo/servo/issues/7396
   // readonly attribute HTMLAllCollection all;
@@ -193,7 +193,7 @@ partial interface Document {
   [LenientSetter] readonly attribute Element? fullscreenElement;
   [LenientSetter] readonly attribute boolean fullscreen; // historical
 
-  Promise<void> exitFullscreen();
+  Promise<undefined> exitFullscreen();
 
   attribute EventHandler onfullscreenchange;
   attribute EventHandler onfullscreenerror;
diff --git a/components/script/dom/webidls/Element.webidl b/components/script/dom/webidls/Element.webidl
index 4c44b2cd431..319964a7e74 100644
--- a/components/script/dom/webidls/Element.webidl
+++ b/components/script/dom/webidls/Element.webidl
@@ -44,13 +44,13 @@ interface Element : Node {
   [CEReactions, Throws]
   boolean toggleAttribute(DOMString name, optional boolean force);
   [CEReactions, Throws]
-  void setAttribute(DOMString name, DOMString value);
+  undefined setAttribute(DOMString name, DOMString value);
   [CEReactions, Throws]
-  void setAttributeNS(DOMString? namespace, DOMString name, DOMString value);
+  undefined setAttributeNS(DOMString? namespace, DOMString name, DOMString value);
   [CEReactions]
-  void removeAttribute(DOMString name);
+  undefined removeAttribute(DOMString name);
   [CEReactions]
-  void removeAttributeNS(DOMString? namespace, DOMString localName);
+  undefined removeAttributeNS(DOMString? namespace, DOMString localName);
   boolean hasAttribute(DOMString name);
   boolean hasAttributeNS(DOMString? namespace, DOMString localName);
 
@@ -79,9 +79,9 @@ interface Element : Node {
   [CEReactions, Throws]
   Element? insertAdjacentElement(DOMString where_, Element element); // historical
   [Throws]
-  void insertAdjacentText(DOMString where_, DOMString data);
+  undefined insertAdjacentText(DOMString where_, DOMString data);
   [CEReactions, Throws]
-  void insertAdjacentHTML(DOMString position, DOMString html);
+  undefined insertAdjacentHTML(DOMString position, DOMString html);
 
   [Throws, Pref="dom.shadowdom.enabled"] ShadowRoot attachShadow();
 };
@@ -92,13 +92,13 @@ partial interface Element {
   [NewObject]
   DOMRect getBoundingClientRect();
 
-  void scroll(optional ScrollToOptions options = {});
-  void scroll(unrestricted double x, unrestricted double y);
+  undefined scroll(optional ScrollToOptions options = {});
+  undefined scroll(unrestricted double x, unrestricted double y);
 
-  void scrollTo(optional ScrollToOptions options = {});
-  void scrollTo(unrestricted double x, unrestricted double y);
-  void scrollBy(optional ScrollToOptions options = {});
-  void scrollBy(unrestricted double x, unrestricted double y);
+  undefined scrollTo(optional ScrollToOptions options = {});
+  undefined scrollTo(unrestricted double x, unrestricted double y);
+  undefined scrollBy(optional ScrollToOptions options = {});
+  undefined scrollBy(unrestricted double x, unrestricted double y);
   attribute unrestricted double scrollTop;
   attribute unrestricted double scrollLeft;
   readonly attribute long scrollWidth;
@@ -120,7 +120,7 @@ partial interface Element {
 
 // https://fullscreen.spec.whatwg.org/#api
 partial interface Element {
-  Promise<void> requestFullscreen();
+  Promise<undefined> requestFullscreen();
 };
 
 Element includes ChildNode;
diff --git a/components/script/dom/webidls/Event.webidl b/components/script/dom/webidls/Event.webidl
index 0c2c122e044..e7888b38fb3 100644
--- a/components/script/dom/webidls/Event.webidl
+++ b/components/script/dom/webidls/Event.webidl
@@ -21,16 +21,16 @@ interface Event {
   const unsigned short BUBBLING_PHASE = 3;
   readonly attribute unsigned short eventPhase;
 
-  void stopPropagation();
+  undefined stopPropagation();
   attribute boolean cancelBubble;
-  void stopImmediatePropagation();
+  undefined stopImmediatePropagation();
 
   [Pure]
   readonly attribute boolean bubbles;
   [Pure]
   readonly attribute boolean cancelable;
   attribute boolean returnValue;  // historical
-  void preventDefault();
+  undefined preventDefault();
   [Pure]
   readonly attribute boolean defaultPrevented;
 
@@ -39,7 +39,7 @@ interface Event {
   [Constant]
   readonly attribute DOMHighResTimeStamp timeStamp;
 
-  void initEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false);
+  undefined initEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false);
 };
 
 dictionary EventInit {
diff --git a/components/script/dom/webidls/EventListener.webidl b/components/script/dom/webidls/EventListener.webidl
index f384e661c63..13ed93be5ad 100644
--- a/components/script/dom/webidls/EventListener.webidl
+++ b/components/script/dom/webidls/EventListener.webidl
@@ -7,6 +7,6 @@
 
 [Exposed=Window]
 callback interface EventListener {
-  void handleEvent(Event event);
+  undefined handleEvent(Event event);
 };
 
diff --git a/components/script/dom/webidls/EventSource.webidl b/components/script/dom/webidls/EventSource.webidl
index 5525970891b..c76885bd2e0 100644
--- a/components/script/dom/webidls/EventSource.webidl
+++ b/components/script/dom/webidls/EventSource.webidl
@@ -22,7 +22,7 @@ interface EventSource : EventTarget {
   attribute EventHandler onopen;
   attribute EventHandler onmessage;
   attribute EventHandler onerror;
-  void close();
+  undefined close();
 };
 
 dictionary EventSourceInit {
diff --git a/components/script/dom/webidls/EventTarget.webidl b/components/script/dom/webidls/EventTarget.webidl
index dd0236c910c..82e8c967483 100644
--- a/components/script/dom/webidls/EventTarget.webidl
+++ b/components/script/dom/webidls/EventTarget.webidl
@@ -8,13 +8,13 @@
 [Exposed=(Window,Worker,Worklet,DissimilarOriginWindow)]
 interface EventTarget {
   [Throws] constructor();
-  void addEventListener(
+  undefined addEventListener(
     DOMString type,
     EventListener? callback,
     optional (AddEventListenerOptions or boolean) options = {}
   );
 
-  void removeEventListener(
+  undefined removeEventListener(
     DOMString type,
     EventListener? callback,
     optional (EventListenerOptions or boolean) options = {}
diff --git a/components/script/dom/webidls/ExtendableEvent.webidl b/components/script/dom/webidls/ExtendableEvent.webidl
index 99cafa305a6..0b030873fc5 100644
--- a/components/script/dom/webidls/ExtendableEvent.webidl
+++ b/components/script/dom/webidls/ExtendableEvent.webidl
@@ -9,7 +9,7 @@
 interface ExtendableEvent : Event {
   [Throws] constructor(DOMString type,
  optional ExtendableEventInit eventInitDict = {});
-  [Throws] void waitUntil(/*Promise<*/any/*>*/ f);
+  [Throws] undefined waitUntil(/*Promise<*/any/*>*/ f);
 };
 
 dictionary ExtendableEventInit : EventInit {
diff --git a/components/script/dom/webidls/FakeXRDevice.webidl b/components/script/dom/webidls/FakeXRDevice.webidl
index 6349d70963b..617b18d12f3 100644
--- a/components/script/dom/webidls/FakeXRDevice.webidl
+++ b/components/script/dom/webidls/FakeXRDevice.webidl
@@ -8,27 +8,27 @@
 interface FakeXRDevice {
   // Sets the values to be used for subsequent
   // requestAnimationFrame() callbacks.
-  [Throws] void setViews(sequence<FakeXRViewInit> views);
+  [Throws] undefined setViews(sequence<FakeXRViewInit> views);
 
-  [Throws] void setViewerOrigin(FakeXRRigidTransformInit origin, optional boolean emulatedPosition = false);
-  void clearViewerOrigin();
+  [Throws] undefined setViewerOrigin(FakeXRRigidTransformInit origin, optional boolean emulatedPosition = false);
+  undefined clearViewerOrigin();
 
-  [Throws] void setFloorOrigin(FakeXRRigidTransformInit origin);
-  void clearFloorOrigin();
+  [Throws] undefined setFloorOrigin(FakeXRRigidTransformInit origin);
+  undefined clearFloorOrigin();
 
   // // Simulates devices focusing and blurring sessions.
-  void simulateVisibilityChange(XRVisibilityState state);
+  undefined simulateVisibilityChange(XRVisibilityState state);
 
   // void setBoundsGeometry(sequence<FakeXRBoundsPoint> boundsCoodinates);
 
   [Throws] FakeXRInputController simulateInputSourceConnection(FakeXRInputSourceInit init);
 
   // behaves as if device was disconnected
-  Promise<void> disconnect();
+  Promise<undefined> disconnect();
 
   // Hit test extensions:
-  [Throws] void setWorld(FakeXRWorldInit world);
-  void clearWorld();
+  [Throws] undefined setWorld(FakeXRWorldInit world);
+  undefined clearWorld();
 };
 
 // https://immersive-web.github.io/webxr/#dom-xrwebgllayer-getviewport
diff --git a/components/script/dom/webidls/FakeXRInputController.webidl b/components/script/dom/webidls/FakeXRInputController.webidl
index b8faad1038a..3fa4e2e02b2 100644
--- a/components/script/dom/webidls/FakeXRInputController.webidl
+++ b/components/script/dom/webidls/FakeXRInputController.webidl
@@ -6,19 +6,22 @@
 
 [Exposed=Window, Pref="dom.webxr.test"]
 interface FakeXRInputController {
-  void setHandedness(XRHandedness handedness);
-  void setTargetRayMode(XRTargetRayMode targetRayMode);
-  void setProfiles(sequence<DOMString> profiles);
-  [Throws] void setGripOrigin(FakeXRRigidTransformInit gripOrigin, optional boolean emulatedPosition = false);
-  void clearGripOrigin();
-  [Throws] void setPointerOrigin(FakeXRRigidTransformInit pointerOrigin, optional boolean emulatedPosition = false);
-
-  void disconnect();
-  void reconnect();
-
-  void startSelection();
-  void endSelection();
-  void simulateSelect();
+  undefined setHandedness(XRHandedness handedness);
+  undefined setTargetRayMode(XRTargetRayMode targetRayMode);
+  undefined setProfiles(sequence<DOMString> profiles);
+  [Throws] undefined setGripOrigin(FakeXRRigidTransformInit gripOrigin, optional boolean emulatedPosition = false);
+  undefined clearGripOrigin();
+  [Throws] undefined setPointerOrigin(
+    FakeXRRigidTransformInit pointerOrigin,
+    optional boolean emulatedPosition = false
+  );
+
+  undefined disconnect();
+  undefined reconnect();
+
+  undefined startSelection();
+  undefined endSelection();
+  undefined simulateSelect();
 
   // void setSupportedButtons(sequence<FakeXRButtonStateInit> supportedButtons);
   // void updateButtonState(FakeXRButtonStateInit buttonState);
diff --git a/components/script/dom/webidls/FileReader.webidl b/components/script/dom/webidls/FileReader.webidl
index a1c01fb716c..414958c625b 100644
--- a/components/script/dom/webidls/FileReader.webidl
+++ b/components/script/dom/webidls/FileReader.webidl
@@ -11,13 +11,13 @@ interface FileReader: EventTarget {
 
   // async read methods
   [Throws]
-  void readAsArrayBuffer(Blob blob);
+  undefined readAsArrayBuffer(Blob blob);
   [Throws]
-  void readAsText(Blob blob, optional DOMString label);
+  undefined readAsText(Blob blob, optional DOMString label);
   [Throws]
-  void readAsDataURL(Blob blob);
+  undefined readAsDataURL(Blob blob);
 
-  void abort();
+  undefined abort();
 
   // states
   const unsigned short EMPTY = 0;
diff --git a/components/script/dom/webidls/FormData.webidl b/components/script/dom/webidls/FormData.webidl
index 4e2f5de04ec..ea07dd4249c 100644
--- a/components/script/dom/webidls/FormData.webidl
+++ b/components/script/dom/webidls/FormData.webidl
@@ -11,13 +11,13 @@ typedef (File or USVString) FormDataEntryValue;
 [Exposed=(Window,Worker)]
 interface FormData {
   [Throws] constructor(optional HTMLFormElement form);
-  void append(USVString name, USVString value);
-  void append(USVString name, Blob value, optional USVString filename);
-  void delete(USVString name);
+  undefined append(USVString name, USVString value);
+  undefined append(USVString name, Blob value, optional USVString filename);
+  undefined delete(USVString name);
   FormDataEntryValue? get(USVString name);
   sequence<FormDataEntryValue> getAll(USVString name);
   boolean has(USVString name);
-  void set(USVString name, USVString value);
-  void set(USVString name, Blob value, optional USVString filename);
+  undefined set(USVString name, USVString value);
+  undefined set(USVString name, Blob value, optional USVString filename);
   iterable<USVString, FormDataEntryValue>;
 };
diff --git a/components/script/dom/webidls/GPUBuffer.webidl b/components/script/dom/webidls/GPUBuffer.webidl
index 2097db233e9..819ebb27bab 100644
--- a/components/script/dom/webidls/GPUBuffer.webidl
+++ b/components/script/dom/webidls/GPUBuffer.webidl
@@ -5,11 +5,11 @@
 // https://gpuweb.github.io/gpuweb/#gpubuffer
 [Exposed=(Window, DedicatedWorker), Serializable, Pref="dom.webgpu.enabled"]
 interface GPUBuffer {
-    Promise<void> mapAsync(GPUMapModeFlags mode, optional GPUSize64 offset = 0, optional GPUSize64 size);
+    Promise<undefined> mapAsync(GPUMapModeFlags mode, optional GPUSize64 offset = 0, optional GPUSize64 size);
     [Throws] ArrayBuffer getMappedRange(optional GPUSize64 offset = 0, optional GPUSize64 size);
-    void unmap();
+    undefined unmap();
 
-    void destroy();
+    undefined destroy();
 };
 GPUBuffer includes GPUObjectBase;
 
diff --git a/components/script/dom/webidls/GPUCommandEncoder.webidl b/components/script/dom/webidls/GPUCommandEncoder.webidl
index 00a8e1ad537..a9395778c80 100644
--- a/components/script/dom/webidls/GPUCommandEncoder.webidl
+++ b/components/script/dom/webidls/GPUCommandEncoder.webidl
@@ -8,24 +8,24 @@ interface GPUCommandEncoder {
     GPURenderPassEncoder beginRenderPass(GPURenderPassDescriptor descriptor);
     GPUComputePassEncoder beginComputePass(optional GPUComputePassDescriptor descriptor = {});
 
-    void copyBufferToBuffer(
+    undefined copyBufferToBuffer(
         GPUBuffer source,
         GPUSize64 sourceOffset,
         GPUBuffer destination,
         GPUSize64 destinationOffset,
         GPUSize64 size);
 
-    void copyBufferToTexture(
+    undefined copyBufferToTexture(
         GPUBufferCopyView source,
         GPUTextureCopyView destination,
         GPUExtent3D copySize);
 
-    void copyTextureToBuffer(
+    undefined copyTextureToBuffer(
         GPUTextureCopyView source,
         GPUBufferCopyView destination,
         GPUExtent3D copySize);
 
-    void copyTextureToTexture(
+    undefined copyTextureToTexture(
         GPUTextureCopyView source,
         GPUTextureCopyView destination,
         GPUExtent3D copySize);
diff --git a/components/script/dom/webidls/GPUComputePassEncoder.webidl b/components/script/dom/webidls/GPUComputePassEncoder.webidl
index f7c0eba2138..8fd3befe54b 100644
--- a/components/script/dom/webidls/GPUComputePassEncoder.webidl
+++ b/components/script/dom/webidls/GPUComputePassEncoder.webidl
@@ -5,16 +5,16 @@
 // https://gpuweb.github.io/gpuweb/#gpucomputepassencoder
 [Exposed=(Window, DedicatedWorker), Serializable, Pref="dom.webgpu.enabled"]
 interface GPUComputePassEncoder {
-    void setPipeline(GPUComputePipeline pipeline);
-    void dispatch(GPUSize32 x, optional GPUSize32 y = 1, optional GPUSize32 z = 1);
-    void dispatchIndirect(GPUBuffer indirectBuffer, GPUSize64 indirectOffset);
+    undefined setPipeline(GPUComputePipeline pipeline);
+    undefined dispatch(GPUSize32 x, optional GPUSize32 y = 1, optional GPUSize32 z = 1);
+    undefined dispatchIndirect(GPUBuffer indirectBuffer, GPUSize64 indirectOffset);
 
     //void beginPipelineStatisticsQuery(GPUQuerySet querySet, GPUSize32 queryIndex);
     //void endPipelineStatisticsQuery();
 
     //void writeTimestamp(GPUQuerySet querySet, GPUSize32 queryIndex);
 
-    void endPass();
+    undefined endPass();
 };
 GPUComputePassEncoder includes GPUObjectBase;
 GPUComputePassEncoder includes GPUProgrammablePassEncoder;
diff --git a/components/script/dom/webidls/GPUProgrammablePassEncoder.webidl b/components/script/dom/webidls/GPUProgrammablePassEncoder.webidl
index 71bfdcb163e..0da804ed091 100644
--- a/components/script/dom/webidls/GPUProgrammablePassEncoder.webidl
+++ b/components/script/dom/webidls/GPUProgrammablePassEncoder.webidl
@@ -5,7 +5,7 @@
 // https://gpuweb.github.io/gpuweb/#gpuprogrammablepassencoder
 [Exposed=(Window, DedicatedWorker)]
 interface mixin GPUProgrammablePassEncoder {
-    void setBindGroup(GPUIndex32 index, GPUBindGroup bindGroup,
+    undefined setBindGroup(GPUIndex32 index, GPUBindGroup bindGroup,
                       optional sequence<GPUBufferDynamicOffset> dynamicOffsets = []);
 
     // void setBindGroup(GPUIndex32 index, GPUBindGroup bindGroup,
diff --git a/components/script/dom/webidls/GPUQueue.webidl b/components/script/dom/webidls/GPUQueue.webidl
index a0fede8415e..c6e7e66cb5d 100644
--- a/components/script/dom/webidls/GPUQueue.webidl
+++ b/components/script/dom/webidls/GPUQueue.webidl
@@ -5,19 +5,19 @@
 // https://gpuweb.github.io/gpuweb/#gpuqueue
 [Exposed=(Window, DedicatedWorker), Serializable, Pref="dom.webgpu.enabled"]
 interface GPUQueue {
-    void submit(sequence<GPUCommandBuffer> commandBuffers);
+    undefined submit(sequence<GPUCommandBuffer> commandBuffers);
 
     //GPUFence createFence(optional GPUFenceDescriptor descriptor = {});
     //void signal(GPUFence fence, GPUFenceValue signalValue);
 
-    [Throws] void writeBuffer(
+    [Throws] undefined writeBuffer(
         GPUBuffer buffer,
         GPUSize64 bufferOffset,
         /*[AllowShared]*/ BufferSource data,
         optional GPUSize64 dataOffset = 0,
         optional GPUSize64 size);
 
-    [Throws] void writeTexture(
+    [Throws] undefined writeTexture(
       GPUTextureCopyView destination,
       /*[AllowShared]*/ BufferSource data,
       GPUTextureDataLayout dataLayout,
diff --git a/components/script/dom/webidls/GPURenderEncoderBase.webidl b/components/script/dom/webidls/GPURenderEncoderBase.webidl
index f0c4532b446..945d59d718a 100644
--- a/components/script/dom/webidls/GPURenderEncoderBase.webidl
+++ b/components/script/dom/webidls/GPURenderEncoderBase.webidl
@@ -5,20 +5,25 @@
 // https://gpuweb.github.io/gpuweb/#gpurenderencoderbase
 [Exposed=(Window, DedicatedWorker)]
 interface mixin GPURenderEncoderBase {
-    void setPipeline(GPURenderPipeline pipeline);
+    undefined setPipeline(GPURenderPipeline pipeline);
 
-    void setIndexBuffer(GPUBuffer buffer, optional GPUSize64 offset = 0, optional GPUSize64 size = 0);
-    void setVertexBuffer(GPUIndex32 slot, GPUBuffer buffer, optional GPUSize64 offset = 0, optional GPUSize64 size = 0);
+    undefined setIndexBuffer(GPUBuffer buffer, optional GPUSize64 offset = 0, optional GPUSize64 size = 0);
+    undefined setVertexBuffer(
+        GPUIndex32 slot,
+        GPUBuffer buffer,
+        optional GPUSize64 offset = 0,
+        optional GPUSize64 size = 0
+    );
 
-    void draw(GPUSize32 vertexCount, optional GPUSize32 instanceCount = 1,
+    undefined draw(GPUSize32 vertexCount, optional GPUSize32 instanceCount = 1,
               optional GPUSize32 firstVertex = 0, optional GPUSize32 firstInstance = 0);
-    void drawIndexed(GPUSize32 indexCount, optional GPUSize32 instanceCount = 1,
+    undefined drawIndexed(GPUSize32 indexCount, optional GPUSize32 instanceCount = 1,
                      optional GPUSize32 firstIndex = 0,
                      optional GPUSignedOffset32 baseVertex = 0,
                      optional GPUSize32 firstInstance = 0);
 
-    void drawIndirect(GPUBuffer indirectBuffer, GPUSize64 indirectOffset);
-    void drawIndexedIndirect(GPUBuffer indirectBuffer, GPUSize64 indirectOffset);
+    undefined drawIndirect(GPUBuffer indirectBuffer, GPUSize64 indirectOffset);
+    undefined drawIndexedIndirect(GPUBuffer indirectBuffer, GPUSize64 indirectOffset);
 };
 
 typedef [EnforceRange] long GPUSignedOffset32;
diff --git a/components/script/dom/webidls/GPURenderPassEncoder.webidl b/components/script/dom/webidls/GPURenderPassEncoder.webidl
index 6261cf89453..65d3fdf7976 100644
--- a/components/script/dom/webidls/GPURenderPassEncoder.webidl
+++ b/components/script/dom/webidls/GPURenderPassEncoder.webidl
@@ -5,15 +5,15 @@
 // https://gpuweb.github.io/gpuweb/#gpurenderpassencoder
 [Exposed=(Window, DedicatedWorker), Pref="dom.webgpu.enabled"]
 interface GPURenderPassEncoder {
-    void setViewport(float x, float y,
+    undefined setViewport(float x, float y,
                      float width, float height,
                      float minDepth, float maxDepth);
 
-    void setScissorRect(GPUIntegerCoordinate x, GPUIntegerCoordinate y,
+    undefined setScissorRect(GPUIntegerCoordinate x, GPUIntegerCoordinate y,
                         GPUIntegerCoordinate width, GPUIntegerCoordinate height);
 
-    void setBlendColor(GPUColor color);
-    void setStencilReference(GPUStencilValue reference);
+    undefined setBlendColor(GPUColor color);
+    undefined setStencilReference(GPUStencilValue reference);
 
     //void beginOcclusionQuery(GPUSize32 queryIndex);
     //void endOcclusionQuery();
@@ -23,8 +23,8 @@ interface GPURenderPassEncoder {
 
     //void writeTimestamp(GPUQuerySet querySet, GPUSize32 queryIndex);
 
-    void executeBundles(sequence<GPURenderBundle> bundles);
-    void endPass();
+    undefined executeBundles(sequence<GPURenderBundle> bundles);
+    undefined endPass();
 };
 GPURenderPassEncoder includes GPUObjectBase;
 GPURenderPassEncoder includes GPUProgrammablePassEncoder;
diff --git a/components/script/dom/webidls/GPUTexture.webidl b/components/script/dom/webidls/GPUTexture.webidl
index 9a02a98d577..ad68a533c6c 100644
--- a/components/script/dom/webidls/GPUTexture.webidl
+++ b/components/script/dom/webidls/GPUTexture.webidl
@@ -7,7 +7,7 @@
 interface GPUTexture {
     GPUTextureView createView(optional GPUTextureViewDescriptor descriptor = {});
 
-    void destroy();
+    undefined destroy();
 };
 GPUTexture includes GPUObjectBase;
 
diff --git a/components/script/dom/webidls/GPUValidationError.webidl b/components/script/dom/webidls/GPUValidationError.webidl
index 8e5d211d981..e62feeb7784 100644
--- a/components/script/dom/webidls/GPUValidationError.webidl
+++ b/components/script/dom/webidls/GPUValidationError.webidl
@@ -17,6 +17,6 @@ enum GPUErrorFilter {
 };
 
 partial interface GPUDevice {
-    void pushErrorScope(GPUErrorFilter filter);
+    undefined pushErrorScope(GPUErrorFilter filter);
     Promise<GPUError?> popErrorScope();
 };
diff --git a/components/script/dom/webidls/HTMLButtonElement.webidl b/components/script/dom/webidls/HTMLButtonElement.webidl
index a221f6669cf..178c7bd3a94 100644
--- a/components/script/dom/webidls/HTMLButtonElement.webidl
+++ b/components/script/dom/webidls/HTMLButtonElement.webidl
@@ -35,7 +35,7 @@ interface HTMLButtonElement : HTMLElement {
   readonly attribute DOMString validationMessage;
   boolean checkValidity();
   boolean reportValidity();
-  void setCustomValidity(DOMString error);
+  undefined setCustomValidity(DOMString error);
 
   readonly attribute NodeList labels;
 };
diff --git a/components/script/dom/webidls/HTMLDialogElement.webidl b/components/script/dom/webidls/HTMLDialogElement.webidl
index f83f2547a72..758839e9015 100644
--- a/components/script/dom/webidls/HTMLDialogElement.webidl
+++ b/components/script/dom/webidls/HTMLDialogElement.webidl
@@ -15,5 +15,5 @@ interface HTMLDialogElement : HTMLElement {
   // [CEReactions]
   // void showModal();
   [CEReactions]
-  void close(optional DOMString returnValue);
+  undefined close(optional DOMString returnValue);
 };
diff --git a/components/script/dom/webidls/HTMLElement.webidl b/components/script/dom/webidls/HTMLElement.webidl
index c743c0a7129..961e79ec405 100644
--- a/components/script/dom/webidls/HTMLElement.webidl
+++ b/components/script/dom/webidls/HTMLElement.webidl
@@ -32,11 +32,11 @@ interface HTMLElement : Element {
   // user interaction
   [CEReactions]
            attribute boolean hidden;
-  void click();
+  undefined click();
   // [CEReactions]
   //         attribute long tabIndex;
-  void focus();
-  void blur();
+  undefined focus();
+  undefined blur();
   // [CEReactions]
   //         attribute DOMString accessKey;
   //readonly attribute DOMString accessKeyLabel;
diff --git a/components/script/dom/webidls/HTMLFieldSetElement.webidl b/components/script/dom/webidls/HTMLFieldSetElement.webidl
index 1257540adb3..8774d75a537 100644
--- a/components/script/dom/webidls/HTMLFieldSetElement.webidl
+++ b/components/script/dom/webidls/HTMLFieldSetElement.webidl
@@ -22,5 +22,5 @@ interface HTMLFieldSetElement : HTMLElement {
   readonly attribute DOMString validationMessage;
   boolean checkValidity();
   boolean reportValidity();
-  void setCustomValidity(DOMString error);
+  undefined setCustomValidity(DOMString error);
 };
diff --git a/components/script/dom/webidls/HTMLFormElement.webidl b/components/script/dom/webidls/HTMLFormElement.webidl
index 8bd872eba95..21d23b68d9b 100644
--- a/components/script/dom/webidls/HTMLFormElement.webidl
+++ b/components/script/dom/webidls/HTMLFormElement.webidl
@@ -34,10 +34,10 @@ interface HTMLFormElement : HTMLElement {
   getter Element? (unsigned long index);
   getter (RadioNodeList or Element) (DOMString name);
 
-  void submit();
-  [Throws] void requestSubmit(optional HTMLElement? submitter = null);
+  undefined submit();
+  [Throws] undefined requestSubmit(optional HTMLElement? submitter = null);
   [CEReactions]
-  void reset();
+  undefined reset();
   boolean checkValidity();
   boolean reportValidity();
 };
diff --git a/components/script/dom/webidls/HTMLInputElement.webidl b/components/script/dom/webidls/HTMLInputElement.webidl
index 6b8c557e83d..3c9f2516717 100644
--- a/components/script/dom/webidls/HTMLInputElement.webidl
+++ b/components/script/dom/webidls/HTMLInputElement.webidl
@@ -79,19 +79,19 @@ interface HTMLInputElement : HTMLElement {
   // [CEReactions]
   //          attribute unsigned long width;
 
-  [Throws] void stepUp(optional long n = 1);
-  [Throws] void stepDown(optional long n = 1);
+  [Throws] undefined stepUp(optional long n = 1);
+  [Throws] undefined stepDown(optional long n = 1);
 
   readonly attribute boolean willValidate;
   readonly attribute ValidityState validity;
   readonly attribute DOMString validationMessage;
   boolean checkValidity();
   boolean reportValidity();
-  void setCustomValidity(DOMString error);
+  undefined setCustomValidity(DOMString error);
 
   readonly attribute NodeList? labels;
 
-  void select();
+  undefined select();
   [SetterThrows]
            attribute unsigned long? selectionStart;
   [SetterThrows]
@@ -99,18 +99,18 @@ interface HTMLInputElement : HTMLElement {
   [SetterThrows]
            attribute DOMString? selectionDirection;
   [Throws]
-           void setRangeText(DOMString replacement);
+           undefined setRangeText(DOMString replacement);
   [Throws]
-           void setRangeText(DOMString replacement, unsigned long start, unsigned long end,
+           undefined setRangeText(DOMString replacement, unsigned long start, unsigned long end,
                              optional SelectionMode selectionMode = "preserve");
   [Throws]
-           void setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
+           undefined setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
 
   // also has obsolete members
 
   // Select with file-system paths for testing purpose
   [Pref="dom.testing.htmlinputelement.select_files.enabled"]
-  void selectFiles(sequence<DOMString> path);
+  undefined selectFiles(sequence<DOMString> path);
 };
 
 // https://html.spec.whatwg.org/multipage/#HTMLInputElement-partial
diff --git a/components/script/dom/webidls/HTMLMediaElement.webidl b/components/script/dom/webidls/HTMLMediaElement.webidl
index fd658913ca5..a2c7866a3fe 100644
--- a/components/script/dom/webidls/HTMLMediaElement.webidl
+++ b/components/script/dom/webidls/HTMLMediaElement.webidl
@@ -24,7 +24,7 @@ interface HTMLMediaElement : HTMLElement {
   readonly attribute unsigned short networkState;
   [CEReactions] attribute DOMString preload;
   readonly attribute TimeRanges buffered;
-  void load();
+  undefined load();
   CanPlayTypeResult canPlayType(DOMString type);
 
   // ready state
@@ -38,7 +38,7 @@ interface HTMLMediaElement : HTMLElement {
 
   // playback state
   attribute double currentTime;
-  void fastSeek(double time);
+  undefined fastSeek(double time);
   readonly attribute unrestricted double duration;
   // Date getStartDate();
   readonly attribute boolean paused;
@@ -49,8 +49,8 @@ interface HTMLMediaElement : HTMLElement {
   readonly attribute boolean ended;
   [CEReactions] attribute boolean autoplay;
   [CEReactions] attribute boolean loop;
-  Promise<void> play();
-  void pause();
+  Promise<undefined> play();
+  undefined pause();
 
   // controls
   [CEReactions] attribute boolean controls;
diff --git a/components/script/dom/webidls/HTMLObjectElement.webidl b/components/script/dom/webidls/HTMLObjectElement.webidl
index 9d60c4a4f92..4c47fcfb93f 100644
--- a/components/script/dom/webidls/HTMLObjectElement.webidl
+++ b/components/script/dom/webidls/HTMLObjectElement.webidl
@@ -30,7 +30,7 @@ interface HTMLObjectElement : HTMLElement {
   readonly attribute DOMString validationMessage;
   boolean checkValidity();
   boolean reportValidity();
-  void setCustomValidity(DOMString error);
+  undefined setCustomValidity(DOMString error);
 
   //legacycaller any (any... arguments);
 
diff --git a/components/script/dom/webidls/HTMLOptionsCollection.webidl b/components/script/dom/webidls/HTMLOptionsCollection.webidl
index 91906b785fb..c2b8baa24d6 100644
--- a/components/script/dom/webidls/HTMLOptionsCollection.webidl
+++ b/components/script/dom/webidls/HTMLOptionsCollection.webidl
@@ -9,10 +9,10 @@ interface HTMLOptionsCollection : HTMLCollection {
   [CEReactions]
   attribute unsigned long length; // shadows inherited length
   [CEReactions, Throws]
-  setter void (unsigned long index, HTMLOptionElement? option);
+  setter undefined (unsigned long index, HTMLOptionElement? option);
   [CEReactions, Throws]
-  void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
+  undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
   [CEReactions]
-  void remove(long index);
+  undefined remove(long index);
   attribute long selectedIndex;
 };
diff --git a/components/script/dom/webidls/HTMLOutputElement.webidl b/components/script/dom/webidls/HTMLOutputElement.webidl
index c44f2a7f44c..403938ad8f8 100644
--- a/components/script/dom/webidls/HTMLOutputElement.webidl
+++ b/components/script/dom/webidls/HTMLOutputElement.webidl
@@ -23,7 +23,7 @@ interface HTMLOutputElement : HTMLElement {
   readonly attribute DOMString validationMessage;
   boolean checkValidity();
   boolean reportValidity();
-  void setCustomValidity(DOMString error);
+  undefined setCustomValidity(DOMString error);
 
   readonly attribute NodeList labels;
 };
diff --git a/components/script/dom/webidls/HTMLSelectElement.webidl b/components/script/dom/webidls/HTMLSelectElement.webidl
index 91258f88ee8..d73c1737c8d 100644
--- a/components/script/dom/webidls/HTMLSelectElement.webidl
+++ b/components/script/dom/webidls/HTMLSelectElement.webidl
@@ -30,12 +30,12 @@ interface HTMLSelectElement : HTMLElement {
   HTMLOptionElement? namedItem(DOMString name);
 
   [CEReactions, Throws]
-  void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
+  undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
   [CEReactions]
-  void remove(); // ChildNode overload
+  undefined remove(); // ChildNode overload
   [CEReactions]
-  void remove(long index);
-  [CEReactions, Throws] setter void (unsigned long index, HTMLOptionElement? option);
+  undefined remove(long index);
+  [CEReactions, Throws] setter undefined (unsigned long index, HTMLOptionElement? option);
 
   // readonly attribute HTMLCollection selectedOptions;
   attribute long selectedIndex;
@@ -46,7 +46,7 @@ interface HTMLSelectElement : HTMLElement {
   readonly attribute DOMString validationMessage;
   boolean checkValidity();
   boolean reportValidity();
-  void setCustomValidity(DOMString error);
+  undefined setCustomValidity(DOMString error);
 
   readonly attribute NodeList labels;
 };
diff --git a/components/script/dom/webidls/HTMLTableElement.webidl b/components/script/dom/webidls/HTMLTableElement.webidl
index 05acced2e57..bc389bfb796 100644
--- a/components/script/dom/webidls/HTMLTableElement.webidl
+++ b/components/script/dom/webidls/HTMLTableElement.webidl
@@ -11,26 +11,26 @@ interface HTMLTableElement : HTMLElement {
            attribute HTMLTableCaptionElement? caption;
   HTMLTableCaptionElement createCaption();
   [CEReactions]
-  void deleteCaption();
+  undefined deleteCaption();
 
   [CEReactions, SetterThrows]
            attribute HTMLTableSectionElement? tHead;
   HTMLTableSectionElement createTHead();
   [CEReactions]
-  void deleteTHead();
+  undefined deleteTHead();
 
   [CEReactions, SetterThrows]
            attribute HTMLTableSectionElement? tFoot;
   HTMLTableSectionElement createTFoot();
   [CEReactions]
-  void deleteTFoot();
+  undefined deleteTFoot();
 
   readonly attribute HTMLCollection tBodies;
   HTMLTableSectionElement createTBody();
 
   readonly attribute HTMLCollection rows;
   [Throws] HTMLTableRowElement insertRow(optional long index = -1);
-  [CEReactions, Throws] void deleteRow(long index);
+  [CEReactions, Throws] undefined deleteRow(long index);
 
   // also has obsolete members
 };
diff --git a/components/script/dom/webidls/HTMLTableRowElement.webidl b/components/script/dom/webidls/HTMLTableRowElement.webidl
index c51e06e6fdc..3f2db6fb720 100644
--- a/components/script/dom/webidls/HTMLTableRowElement.webidl
+++ b/components/script/dom/webidls/HTMLTableRowElement.webidl
@@ -13,7 +13,7 @@ interface HTMLTableRowElement : HTMLElement {
   [Throws]
   HTMLElement insertCell(optional long index = -1);
   [CEReactions, Throws]
-  void deleteCell(long index);
+  undefined deleteCell(long index);
 
   // also has obsolete members
 };
diff --git a/components/script/dom/webidls/HTMLTableSectionElement.webidl b/components/script/dom/webidls/HTMLTableSectionElement.webidl
index e73d16020dc..acb8f89865e 100644
--- a/components/script/dom/webidls/HTMLTableSectionElement.webidl
+++ b/components/script/dom/webidls/HTMLTableSectionElement.webidl
@@ -11,7 +11,7 @@ interface HTMLTableSectionElement : HTMLElement {
   [Throws]
   HTMLElement insertRow(optional long index = -1);
   [CEReactions, Throws]
-  void deleteRow(long index);
+  undefined deleteRow(long index);
 
   // also has obsolete members
 };
diff --git a/components/script/dom/webidls/HTMLTextAreaElement.webidl b/components/script/dom/webidls/HTMLTextAreaElement.webidl
index b726946bc48..e9ffbd0a9e6 100644
--- a/components/script/dom/webidls/HTMLTextAreaElement.webidl
+++ b/components/script/dom/webidls/HTMLTextAreaElement.webidl
@@ -48,11 +48,11 @@ interface HTMLTextAreaElement : HTMLElement {
   readonly attribute DOMString validationMessage;
   boolean checkValidity();
   boolean reportValidity();
-  void setCustomValidity(DOMString error);
+  undefined setCustomValidity(DOMString error);
 
   readonly attribute NodeList labels;
 
-  void select();
+  undefined select();
   [SetterThrows]
            attribute unsigned long? selectionStart;
   [SetterThrows]
@@ -60,10 +60,10 @@ interface HTMLTextAreaElement : HTMLElement {
   [SetterThrows]
            attribute DOMString? selectionDirection;
   [Throws]
-           void setRangeText(DOMString replacement);
+           undefined setRangeText(DOMString replacement);
   [Throws]
-           void setRangeText(DOMString replacement, unsigned long start, unsigned long end,
+           undefined setRangeText(DOMString replacement, unsigned long start, unsigned long end,
                              optional SelectionMode selectionMode = "preserve");
   [Throws]
-           void setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
+           undefined setSelectionRange(unsigned long start, unsigned long end, optional DOMString direction);
 };
diff --git a/components/script/dom/webidls/Headers.webidl b/components/script/dom/webidls/Headers.webidl
index dac3d860154..16e35060b90 100644
--- a/components/script/dom/webidls/Headers.webidl
+++ b/components/script/dom/webidls/Headers.webidl
@@ -10,14 +10,14 @@ typedef (sequence<sequence<ByteString>> or record<ByteString, ByteString>) Heade
 interface Headers {
   [Throws] constructor(optional HeadersInit init);
   [Throws]
-  void append(ByteString name, ByteString value);
+  undefined append(ByteString name, ByteString value);
   [Throws]
-  void delete(ByteString name);
+  undefined delete(ByteString name);
   [Throws]
   ByteString? get(ByteString name);
   [Throws]
   boolean has(ByteString name);
   [Throws]
-  void set(ByteString name, ByteString value);
+  undefined set(ByteString name, ByteString value);
   iterable<ByteString, ByteString>;
 };
diff --git a/components/script/dom/webidls/History.webidl b/components/script/dom/webidls/History.webidl
index 95248cbcd9b..b54ec98723f 100644
--- a/components/script/dom/webidls/History.webidl
+++ b/components/script/dom/webidls/History.webidl
@@ -14,13 +14,13 @@ interface History {
   [Throws]
   readonly attribute any state;
   [Throws]
-  void go(optional long delta = 0);
+  undefined go(optional long delta = 0);
   [Throws]
-  void back();
+  undefined back();
   [Throws]
-  void forward();
+  undefined forward();
   [Throws]
-  void pushState(any data, DOMString title, optional USVString? url = null);
+  undefined pushState(any data, DOMString title, optional USVString? url = null);
   [Throws]
-  void replaceState(any data, DOMString title, optional USVString? url = null);
+  undefined replaceState(any data, DOMString title, optional USVString? url = null);
 };
diff --git a/components/script/dom/webidls/KeyboardEvent.webidl b/components/script/dom/webidls/KeyboardEvent.webidl
index 6933a0105ed..9bddcb03a56 100644
--- a/components/script/dom/webidls/KeyboardEvent.webidl
+++ b/components/script/dom/webidls/KeyboardEvent.webidl
@@ -30,7 +30,7 @@ interface KeyboardEvent : UIEvent {
 // https://w3c.github.io/uievents/#idl-interface-KeyboardEvent-initializers
 partial interface KeyboardEvent {
     // Originally introduced (and deprecated) in DOM Level 3
-    void initKeyboardEvent (DOMString typeArg, boolean bubblesArg, boolean cancelableArg, Window? viewArg,
+    undefined initKeyboardEvent (DOMString typeArg, boolean bubblesArg, boolean cancelableArg, Window? viewArg,
                             DOMString keyArg, unsigned long locationArg, DOMString modifiersListArg,
                             boolean repeat, DOMString locale);
 };
diff --git a/components/script/dom/webidls/Location.webidl b/components/script/dom/webidls/Location.webidl
index 4d9fe2239f2..b56cc220932 100644
--- a/components/script/dom/webidls/Location.webidl
+++ b/components/script/dom/webidls/Location.webidl
@@ -15,10 +15,10 @@
   [Throws]          attribute USVString search;
   [Throws]          attribute USVString hash;
 
-  [Throws] void assign(USVString url);
+  [Throws] undefined assign(USVString url);
   [Throws, CrossOriginCallable]
-           void replace(USVString url);
-  [Throws] void reload();
+           undefined replace(USVString url);
+  [Throws] undefined reload();
 
   //[SameObject] readonly attribute USVString[] ancestorOrigins;
 };
diff --git a/components/script/dom/webidls/MediaList.webidl b/components/script/dom/webidls/MediaList.webidl
index b2ba10f0d7a..e3c9cd6e680 100644
--- a/components/script/dom/webidls/MediaList.webidl
+++ b/components/script/dom/webidls/MediaList.webidl
@@ -8,6 +8,6 @@ interface MediaList {
   stringifier attribute [TreatNullAs=EmptyString] DOMString mediaText;
   readonly attribute unsigned long length;
   getter DOMString? item(unsigned long index);
-  void appendMedium(DOMString medium);
-  void deleteMedium(DOMString medium);
+  undefined appendMedium(DOMString medium);
+  undefined deleteMedium(DOMString medium);
 };
diff --git a/components/script/dom/webidls/MediaQueryList.webidl b/components/script/dom/webidls/MediaQueryList.webidl
index 3818382645a..2a5c0ac2db0 100644
--- a/components/script/dom/webidls/MediaQueryList.webidl
+++ b/components/script/dom/webidls/MediaQueryList.webidl
@@ -8,7 +8,7 @@
 interface MediaQueryList : EventTarget {
   readonly attribute DOMString media;
   readonly attribute boolean matches;
-  void addListener(EventListener? listener);
-  void removeListener(EventListener? listener);
+  undefined addListener(EventListener? listener);
+  undefined removeListener(EventListener? listener);
            attribute EventHandler onchange;
 };
diff --git a/components/script/dom/webidls/MediaSession.webidl b/components/script/dom/webidls/MediaSession.webidl
index 2680ea0c40b..f8954fb4a06 100644
--- a/components/script/dom/webidls/MediaSession.webidl
+++ b/components/script/dom/webidls/MediaSession.webidl
@@ -48,7 +48,7 @@ dictionary MediaPositionState {
   double position;
 };
 
-callback MediaSessionActionHandler = void(/*MediaSessionActionDetails details*/);
+callback MediaSessionActionHandler = undefined(/*MediaSessionActionDetails details*/);
 
 [Exposed=Window]
 interface MediaSession {
@@ -56,7 +56,7 @@ interface MediaSession {
 
   attribute MediaSessionPlaybackState playbackState;
 
-  void setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);
+  undefined setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);
 
-  [Throws] void setPositionState(optional MediaPositionState state = {});
+  [Throws] undefined setPositionState(optional MediaPositionState state = {});
 };
diff --git a/components/script/dom/webidls/MediaStream.webidl b/components/script/dom/webidls/MediaStream.webidl
index 3df07a22454..0fd5a6a846d 100644
--- a/components/script/dom/webidls/MediaStream.webidl
+++ b/components/script/dom/webidls/MediaStream.webidl
@@ -14,8 +14,8 @@ interface MediaStream : EventTarget {
     sequence<MediaStreamTrack> getVideoTracks();
     sequence<MediaStreamTrack> getTracks();
     MediaStreamTrack? getTrackById(DOMString trackId);
-    void addTrack(MediaStreamTrack track);
-    void removeTrack(MediaStreamTrack track);
+    undefined addTrack(MediaStreamTrack track);
+    undefined removeTrack(MediaStreamTrack track);
     MediaStream clone();
     // readonly        attribute boolean active;
     //                 attribute EventHandler onaddtrack;
diff --git a/components/script/dom/webidls/MessageEvent.webidl b/components/script/dom/webidls/MessageEvent.webidl
index b37d725b793..3b546cc1b93 100644
--- a/components/script/dom/webidls/MessageEvent.webidl
+++ b/components/script/dom/webidls/MessageEvent.webidl
@@ -12,7 +12,7 @@ interface MessageEvent : Event {
   readonly attribute MessageEventSource? source;
   readonly attribute /*FrozenArray<MessagePort>*/any ports;
 
-  void initMessageEvent(
+  undefined initMessageEvent(
     DOMString type,
     optional boolean bubbles = false,
     optional boolean cancelable = false,
diff --git a/components/script/dom/webidls/MessagePort.webidl b/components/script/dom/webidls/MessagePort.webidl
index 58a3f06a960..2ba531ce5b2 100644
--- a/components/script/dom/webidls/MessagePort.webidl
+++ b/components/script/dom/webidls/MessagePort.webidl
@@ -8,10 +8,10 @@
 
 [Exposed=(Window,Worker)]
 interface MessagePort : EventTarget {
-  [Throws] void postMessage(any message, sequence<object> transfer);
-  [Throws] void postMessage(any message, optional PostMessageOptions options = {});
-  void start();
-  void close();
+  [Throws] undefined postMessage(any message, sequence<object> transfer);
+  [Throws] undefined postMessage(any message, optional PostMessageOptions options = {});
+  undefined start();
+  undefined close();
 
   // event handlers
   attribute EventHandler onmessage;
diff --git a/components/script/dom/webidls/MouseEvent.webidl b/components/script/dom/webidls/MouseEvent.webidl
index 0b7cb644368..ba717f724a4 100644
--- a/components/script/dom/webidls/MouseEvent.webidl
+++ b/components/script/dom/webidls/MouseEvent.webidl
@@ -44,7 +44,7 @@ dictionary MouseEventInit : EventModifierInit {
 // https://w3c.github.io/uievents/#idl-interface-MouseEvent-initializers
 partial interface MouseEvent {
     // Deprecated in DOM Level 3
-    void initMouseEvent (DOMString typeArg, boolean bubblesArg, boolean cancelableArg,
+    undefined initMouseEvent (DOMString typeArg, boolean bubblesArg, boolean cancelableArg,
                          Window? viewArg, long detailArg,
                          long screenXArg, long screenYArg,
                          long clientXArg, long clientYArg,
diff --git a/components/script/dom/webidls/MutationObserver.webidl b/components/script/dom/webidls/MutationObserver.webidl
index 7d75c5ed0c4..baa9a1a79b0 100644
--- a/components/script/dom/webidls/MutationObserver.webidl
+++ b/components/script/dom/webidls/MutationObserver.webidl
@@ -11,12 +11,12 @@
 interface MutationObserver {
     [Throws] constructor(MutationCallback callback);
     [Throws]
-    void observe(Node target, optional MutationObserverInit options = {});
-    void disconnect();
+    undefined observe(Node target, optional MutationObserverInit options = {});
+    undefined disconnect();
     sequence<MutationRecord> takeRecords();
 };
 
-callback MutationCallback = void (sequence<MutationRecord> mutations, MutationObserver observer);
+callback MutationCallback = undefined (sequence<MutationRecord> mutations, MutationObserver observer);
 
 dictionary MutationObserverInit {
     boolean childList = false;
diff --git a/components/script/dom/webidls/NavigationPreloadManager.webidl b/components/script/dom/webidls/NavigationPreloadManager.webidl
index ba8d329769e..1c2e4d09189 100644
--- a/components/script/dom/webidls/NavigationPreloadManager.webidl
+++ b/components/script/dom/webidls/NavigationPreloadManager.webidl
@@ -5,9 +5,9 @@
 // https://w3c.github.io/ServiceWorker/#navigation-preload-manager
 [Pref="dom.serviceworker.enabled", SecureContext, Exposed=(Window,Worker)]
 interface NavigationPreloadManager {
-  Promise<void> enable();
-  Promise<void> disable();
-  Promise<void> setHeaderValue(ByteString value);
+  Promise<undefined> enable();
+  Promise<undefined> disable();
+  Promise<undefined> setHeaderValue(ByteString value);
   Promise<NavigationPreloadState> getState();
 };
 
diff --git a/components/script/dom/webidls/Node.webidl b/components/script/dom/webidls/Node.webidl
index c8a71dbebb7..0e5624a34f9 100644
--- a/components/script/dom/webidls/Node.webidl
+++ b/components/script/dom/webidls/Node.webidl
@@ -59,7 +59,7 @@ interface Node : EventTarget {
   [CEReactions, Pure]
            attribute DOMString? textContent;
   [CEReactions]
-  void normalize();
+  undefined normalize();
 
   [CEReactions, Throws]
   Node cloneNode(optional boolean deep = false);
diff --git a/components/script/dom/webidls/NodeIterator.webidl b/components/script/dom/webidls/NodeIterator.webidl
index 9c9e1f41e80..ca433fbcfe7 100644
--- a/components/script/dom/webidls/NodeIterator.webidl
+++ b/components/script/dom/webidls/NodeIterator.webidl
@@ -25,5 +25,5 @@ interface NodeIterator {
   Node? previousNode();
 
   [Pure]
-  void detach();
+  undefined detach();
 };
diff --git a/components/script/dom/webidls/OESVertexArrayObject.webidl b/components/script/dom/webidls/OESVertexArrayObject.webidl
index 21c59fd7a88..0996ef45d91 100644
--- a/components/script/dom/webidls/OESVertexArrayObject.webidl
+++ b/components/script/dom/webidls/OESVertexArrayObject.webidl
@@ -11,7 +11,7 @@ interface OESVertexArrayObject {
     const unsigned long VERTEX_ARRAY_BINDING_OES = 0x85B5;
 
     WebGLVertexArrayObjectOES? createVertexArrayOES();
-    void deleteVertexArrayOES(WebGLVertexArrayObjectOES? arrayObject);
+    undefined deleteVertexArrayOES(WebGLVertexArrayObjectOES? arrayObject);
     boolean isVertexArrayOES(WebGLVertexArrayObjectOES? arrayObject);
-    void bindVertexArrayOES(WebGLVertexArrayObjectOES? arrayObject);
+    undefined bindVertexArrayOES(WebGLVertexArrayObjectOES? arrayObject);
 };
diff --git a/components/script/dom/webidls/PaintWorkletGlobalScope.webidl b/components/script/dom/webidls/PaintWorkletGlobalScope.webidl
index b80bf92dce5..981df09321f 100644
--- a/components/script/dom/webidls/PaintWorkletGlobalScope.webidl
+++ b/components/script/dom/webidls/PaintWorkletGlobalScope.webidl
@@ -5,9 +5,9 @@
 // https://drafts.css-houdini.org/css-paint-api/#paintworkletglobalscope
 [Global=(Worklet,PaintWorklet), Pref="dom.worklet.enabled", Exposed=PaintWorklet]
 interface PaintWorkletGlobalScope : WorkletGlobalScope {
-    [Throws] void registerPaint(DOMString name, VoidFunction paintCtor);
+    [Throws] undefined registerPaint(DOMString name, VoidFunction paintCtor);
     // This function is to be used only for testing, and should not be
     // accessible outside of that use.
     [Pref="dom.worklet.blockingsleep.enabled"]
-    void sleep(unsigned long long ms);
+    undefined sleep(unsigned long long ms);
 };
diff --git a/components/script/dom/webidls/PannerNode.webidl b/components/script/dom/webidls/PannerNode.webidl
index c692a97e278..a5119040b64 100644
--- a/components/script/dom/webidls/PannerNode.webidl
+++ b/components/script/dom/webidls/PannerNode.webidl
@@ -51,6 +51,6 @@ interface PannerNode : AudioNode {
   attribute double coneInnerAngle;
   attribute double coneOuterAngle;
   [SetterThrows] attribute double coneOuterGain;
-  void setPosition (float x, float y, float z);
-  void setOrientation (float x, float y, float z);
+  undefined setPosition (float x, float y, float z);
+  undefined setOrientation (float x, float y, float z);
 };
diff --git a/components/script/dom/webidls/ParentNode.webidl b/components/script/dom/webidls/ParentNode.webidl
index a7bcc64cd23..3a35b77dc13 100644
--- a/components/script/dom/webidls/ParentNode.webidl
+++ b/components/script/dom/webidls/ParentNode.webidl
@@ -17,11 +17,11 @@ interface mixin ParentNode {
   readonly attribute unsigned long childElementCount;
 
   [CEReactions, Throws, Unscopable]
-  void prepend((Node or DOMString)... nodes);
+  undefined prepend((Node or DOMString)... nodes);
   [CEReactions, Throws, Unscopable]
-  void append((Node or DOMString)... nodes);
+  undefined append((Node or DOMString)... nodes);
   [CEReactions, Throws, Unscopable]
-  void replaceChildren((Node or DOMString)... nodes);
+  undefined replaceChildren((Node or DOMString)... nodes);
 
   [Pure, Throws]
   Element? querySelector(DOMString selectors);
diff --git a/components/script/dom/webidls/Performance.webidl b/components/script/dom/webidls/Performance.webidl
index 5db6551ff96..46f751cb4ab 100644
--- a/components/script/dom/webidls/Performance.webidl
+++ b/components/script/dom/webidls/Performance.webidl
@@ -29,17 +29,17 @@ partial interface Performance {
 [Exposed=(Window,Worker)]
 partial interface Performance {
   [Throws]
-  void mark(DOMString markName);
-  void clearMarks(optional DOMString markName);
+  undefined mark(DOMString markName);
+  undefined clearMarks(optional DOMString markName);
   [Throws]
-  void measure(DOMString measureName, optional DOMString startMark, optional DOMString endMark);
-  void clearMeasures(optional DOMString measureName);
+  undefined measure(DOMString measureName, optional DOMString startMark, optional DOMString endMark);
+  undefined clearMeasures(optional DOMString measureName);
 };
 
 //https://w3c.github.io/resource-timing/#sec-extensions-performance-interface
 partial interface Performance {
-  void clearResourceTimings ();
-  void setResourceTimingBufferSize (unsigned long maxSize);
+  undefined clearResourceTimings ();
+  undefined setResourceTimingBufferSize (unsigned long maxSize);
               attribute EventHandler onresourcetimingbufferfull;
 };
 
diff --git a/components/script/dom/webidls/PerformanceObserver.webidl b/components/script/dom/webidls/PerformanceObserver.webidl
index db511a7b4b7..806020eb70f 100644
--- a/components/script/dom/webidls/PerformanceObserver.webidl
+++ b/components/script/dom/webidls/PerformanceObserver.webidl
@@ -12,14 +12,14 @@ dictionary PerformanceObserverInit {
   boolean buffered;
 };
 
-callback PerformanceObserverCallback = void (PerformanceObserverEntryList entries, PerformanceObserver observer);
+callback PerformanceObserverCallback = undefined (PerformanceObserverEntryList entries, PerformanceObserver observer);
 
 [Exposed=(Window,Worker)]
 interface PerformanceObserver {
   [Throws] constructor(PerformanceObserverCallback callback);
   [Throws]
-  void observe(optional PerformanceObserverInit options = {});
-  void disconnect();
+  undefined observe(optional PerformanceObserverInit options = {});
+  undefined disconnect();
   PerformanceEntryList takeRecords();
   // codegen doesn't like SameObject+static and doesn't know FrozenArray
   /*[SameObject]*/ static readonly attribute /*FrozenArray<DOMString>*/ any supportedEntryTypes;
diff --git a/components/script/dom/webidls/PluginArray.webidl b/components/script/dom/webidls/PluginArray.webidl
index 09883cbfad4..53abc12b027 100644
--- a/components/script/dom/webidls/PluginArray.webidl
+++ b/components/script/dom/webidls/PluginArray.webidl
@@ -5,7 +5,7 @@
 // https://html.spec.whatwg.org/multipage/#pluginarray
 [LegacyUnenumerableNamedProperties, Exposed=Window]
 interface PluginArray {
-  void refresh(optional boolean reload = false);
+  undefined refresh(optional boolean reload = false);
   readonly attribute unsigned long length;
   getter Plugin? item(unsigned long index);
   getter Plugin? namedItem(DOMString name);
diff --git a/components/script/dom/webidls/Promise.webidl b/components/script/dom/webidls/Promise.webidl
index 6cfbfb68bb0..f4f6616f107 100644
--- a/components/script/dom/webidls/Promise.webidl
+++ b/components/script/dom/webidls/Promise.webidl
@@ -5,7 +5,7 @@
 // This interface is entirely internal to Servo, and should not be accessible to
 // web pages.
 
-callback PromiseJobCallback = void();
+callback PromiseJobCallback = undefined();
 
 [TreatNonCallableAsNull]
 callback AnyCallback = any (any value);
diff --git a/components/script/dom/webidls/RTCDataChannel.webidl b/components/script/dom/webidls/RTCDataChannel.webidl
index cdc3b9fdd48..e67c299bcea 100644
--- a/components/script/dom/webidls/RTCDataChannel.webidl
+++ b/components/script/dom/webidls/RTCDataChannel.webidl
@@ -21,13 +21,13 @@ interface RTCDataChannel : EventTarget {
   attribute EventHandler onerror;
   attribute EventHandler onclosing;
   attribute EventHandler onclose;
-  void close();
+  undefined close();
   attribute EventHandler onmessage;
   [SetterThrows] attribute DOMString binaryType;
-  [Throws] void send(USVString data);
-  [Throws] void send(Blob data);
-  [Throws] void send(ArrayBuffer data);
-  [Throws] void send(ArrayBufferView data);
+  [Throws] undefined send(USVString data);
+  [Throws] undefined send(Blob data);
+  [Throws] undefined send(ArrayBuffer data);
+  [Throws] undefined send(ArrayBufferView data);
 };
 
 // https://www.w3.org/TR/webrtc/#dom-rtcdatachannelinit
diff --git a/components/script/dom/webidls/RTCPeerConnection.webidl b/components/script/dom/webidls/RTCPeerConnection.webidl
index d75f2f9c14c..85a7297fce1 100644
--- a/components/script/dom/webidls/RTCPeerConnection.webidl
+++ b/components/script/dom/webidls/RTCPeerConnection.webidl
@@ -9,15 +9,15 @@ interface RTCPeerConnection : EventTarget {
     [Throws] constructor(optional RTCConfiguration configuration = {});
     Promise<RTCSessionDescriptionInit> createOffer(optional RTCOfferOptions options = {});
     Promise<RTCSessionDescriptionInit> createAnswer(optional RTCAnswerOptions options = {});
-    Promise<void>                      setLocalDescription(RTCSessionDescriptionInit description);
+    Promise<undefined>                      setLocalDescription(RTCSessionDescriptionInit description);
     readonly attribute RTCSessionDescription? localDescription;
     // readonly attribute RTCSessionDescription? currentLocalDescription;
     // readonly attribute RTCSessionDescription? pendingLocalDescription;
-    Promise<void>                      setRemoteDescription(RTCSessionDescriptionInit description);
+    Promise<undefined>                      setRemoteDescription(RTCSessionDescriptionInit description);
     readonly attribute RTCSessionDescription? remoteDescription;
     // readonly attribute RTCSessionDescription? currentRemoteDescription;
     // readonly attribute RTCSessionDescription? pendingRemoteDescription;
-    Promise<void>                      addIceCandidate(optional RTCIceCandidateInit candidate = {});
+    Promise<undefined>                      addIceCandidate(optional RTCIceCandidateInit candidate = {});
     readonly attribute RTCSignalingState      signalingState;
     readonly attribute RTCIceGatheringState   iceGatheringState;
     readonly attribute RTCIceConnectionState  iceConnectionState;
@@ -26,7 +26,7 @@ interface RTCPeerConnection : EventTarget {
     // static sequence<RTCIceServer>      getDefaultIceServers();
     // RTCConfiguration                   getConfiguration();
     // void                               setConfiguration(RTCConfiguration configuration);
-    void                               close();
+    undefined                               close();
              attribute EventHandler           onnegotiationneeded;
              attribute EventHandler           onicecandidate;
     //          attribute EventHandler           onicecandidateerror;
@@ -36,7 +36,7 @@ interface RTCPeerConnection : EventTarget {
     //          attribute EventHandler           onconnectionstatechange;
 
     // removed from spec, but still shipped by browsers
-    void addStream (MediaStream stream);
+    undefined addStream (MediaStream stream);
 };
 
 dictionary RTCConfiguration {
diff --git a/components/script/dom/webidls/RTCRtpSender.webidl b/components/script/dom/webidls/RTCRtpSender.webidl
index d804b1bb3b7..752ab7682b8 100644
--- a/components/script/dom/webidls/RTCRtpSender.webidl
+++ b/components/script/dom/webidls/RTCRtpSender.webidl
@@ -39,7 +39,7 @@ interface RTCRtpSender {
   //readonly attribute MediaStreamTrack? track;
   //readonly attribute RTCDtlsTransport? transport;
   //static RTCRtpCapabilities? getCapabilities(DOMString kind);
-  Promise<void> setParameters(RTCRtpSendParameters parameters);
+  Promise<undefined> setParameters(RTCRtpSendParameters parameters);
   RTCRtpSendParameters getParameters();
   //Promise<void> replaceTrack(MediaStreamTrack? withTrack);
   //void setStreams(MediaStream... streams);
diff --git a/components/script/dom/webidls/Range.webidl b/components/script/dom/webidls/Range.webidl
index 84cd80da0ef..037093a0762 100644
--- a/components/script/dom/webidls/Range.webidl
+++ b/components/script/dom/webidls/Range.webidl
@@ -25,22 +25,22 @@ interface Range {
   readonly attribute Node commonAncestorContainer;
 
   [Throws]
-  void setStart(Node refNode, unsigned long offset);
+  undefined setStart(Node refNode, unsigned long offset);
   [Throws]
-  void setEnd(Node refNode, unsigned long offset);
+  undefined setEnd(Node refNode, unsigned long offset);
   [Throws]
-  void setStartBefore(Node refNode);
+  undefined setStartBefore(Node refNode);
   [Throws]
-  void setStartAfter(Node refNode);
+  undefined setStartAfter(Node refNode);
   [Throws]
-  void setEndBefore(Node refNode);
+  undefined setEndBefore(Node refNode);
   [Throws]
-  void setEndAfter(Node refNode);
-  void collapse(optional boolean toStart = false);
+  undefined setEndAfter(Node refNode);
+  undefined collapse(optional boolean toStart = false);
   [Throws]
-  void selectNode(Node refNode);
+  undefined selectNode(Node refNode);
   [Throws]
-  void selectNodeContents(Node refNode);
+  undefined selectNodeContents(Node refNode);
 
   const unsigned short START_TO_START = 0;
   const unsigned short START_TO_END = 1;
@@ -49,20 +49,20 @@ interface Range {
   [Pure, Throws]
   short compareBoundaryPoints(unsigned short how, Range sourceRange);
   [CEReactions, Throws]
-  void deleteContents();
+  undefined deleteContents();
   [CEReactions, NewObject, Throws]
   DocumentFragment extractContents();
   [CEReactions, NewObject, Throws]
   DocumentFragment cloneContents();
   [CEReactions, Throws]
-  void insertNode(Node node);
+  undefined insertNode(Node node);
   [CEReactions, Throws]
-  void surroundContents(Node newParent);
+  undefined surroundContents(Node newParent);
 
   [NewObject]
   Range cloneRange();
   [Pure]
-  void detach();
+  undefined detach();
 
   [Pure, Throws]
   boolean isPointInRange(Node node, unsigned long offset);
diff --git a/components/script/dom/webidls/Selection.webidl b/components/script/dom/webidls/Selection.webidl
index 38c874e3bf7..b51181286c1 100644
--- a/components/script/dom/webidls/Selection.webidl
+++ b/components/script/dom/webidls/Selection.webidl
@@ -13,20 +13,20 @@ readonly attribute Node? anchorNode;
   readonly attribute unsigned long rangeCount;
   readonly attribute DOMString type;
   [Throws] Range getRangeAt(unsigned long index);
-  void addRange(Range range);
-  [Throws] void removeRange(Range range);
-  void removeAllRanges();
-  void empty();
-  [Throws] void collapse(Node? node, optional unsigned long offset = 0);
-  [Throws] void setPosition(Node? node, optional unsigned long offset = 0);
-  [Throws] void collapseToStart();
-  [Throws] void collapseToEnd();
-  [Throws] void extend(Node node, optional unsigned long offset = 0);
+  undefined addRange(Range range);
+  [Throws] undefined removeRange(Range range);
+  undefined removeAllRanges();
+  undefined empty();
+  [Throws] undefined collapse(Node? node, optional unsigned long offset = 0);
+  [Throws] undefined setPosition(Node? node, optional unsigned long offset = 0);
+  [Throws] undefined collapseToStart();
+  [Throws] undefined collapseToEnd();
+  [Throws] undefined extend(Node node, optional unsigned long offset = 0);
   [Throws]
-  void setBaseAndExtent(Node anchorNode, unsigned long anchorOffset, Node focusNode, unsigned long focusOffset);
-  [Throws] void selectAllChildren(Node node);
+  undefined setBaseAndExtent(Node anchorNode, unsigned long anchorOffset, Node focusNode, unsigned long focusOffset);
+  [Throws] undefined selectAllChildren(Node node);
   [CEReactions, Throws]
-  void deleteFromDocument();
+  undefined deleteFromDocument();
   boolean containsNode(Node node, optional boolean allowPartialContainment = false);
   stringifier DOMString ();
 };
diff --git a/components/script/dom/webidls/ServiceWorker.webidl b/components/script/dom/webidls/ServiceWorker.webidl
index 60bb6dfc4b9..b91a4c55b51 100644
--- a/components/script/dom/webidls/ServiceWorker.webidl
+++ b/components/script/dom/webidls/ServiceWorker.webidl
@@ -7,8 +7,8 @@
 interface ServiceWorker : EventTarget {
   readonly attribute USVString scriptURL;
   readonly attribute ServiceWorkerState state;
-  [Throws] void postMessage(any message, sequence<object> transfer);
-  [Throws] void postMessage(any message, optional PostMessageOptions options = {});
+  [Throws] undefined postMessage(any message, sequence<object> transfer);
+  [Throws] undefined postMessage(any message, optional PostMessageOptions options = {});
 
   // event
   attribute EventHandler onstatechange;
diff --git a/components/script/dom/webidls/Storage.webidl b/components/script/dom/webidls/Storage.webidl
index 380eb72d265..a4912e35f5f 100644
--- a/components/script/dom/webidls/Storage.webidl
+++ b/components/script/dom/webidls/Storage.webidl
@@ -17,9 +17,9 @@ interface Storage {
   getter DOMString? getItem(DOMString name);
 
   [Throws]
-  setter void setItem(DOMString name, DOMString value);
+  setter undefined setItem(DOMString name, DOMString value);
 
-  deleter void removeItem(DOMString name);
+  deleter undefined removeItem(DOMString name);
 
-  void clear();
+  undefined clear();
 };
diff --git a/components/script/dom/webidls/StorageEvent.webidl b/components/script/dom/webidls/StorageEvent.webidl
index dbc60b23036..42ce914e49f 100644
--- a/components/script/dom/webidls/StorageEvent.webidl
+++ b/components/script/dom/webidls/StorageEvent.webidl
@@ -19,7 +19,7 @@ interface StorageEvent : Event {
   readonly attribute Storage? storageArea;
 
 
-  void initStorageEvent(DOMString type, optional boolean bubbles = false,
+  undefined initStorageEvent(DOMString type, optional boolean bubbles = false,
   optional boolean cancelable = false, optional DOMString? key = null, optional
   DOMString? oldValue = null, optional DOMString? newValue = null, optional
   USVString url = "", optional Storage? storageArea = null);
diff --git a/components/script/dom/webidls/TestBinding.webidl b/components/script/dom/webidls/TestBinding.webidl
index 14ab146d8be..7b105f238ab 100644
--- a/components/script/dom/webidls/TestBinding.webidl
+++ b/components/script/dom/webidls/TestBinding.webidl
@@ -176,8 +176,8 @@ interface TestBinding {
   [PutForwards=booleanAttribute]
   readonly attribute TestBinding forwardedAttribute;
 
-  [BinaryName="BinaryRenamedMethod"] void methToBinaryRename();
-  void receiveVoid();
+  [BinaryName="BinaryRenamedMethod"] undefined methToBinaryRename();
+  undefined receiveVoid();
   boolean receiveBoolean();
   byte receiveByte();
   octet receiveOctet();
@@ -243,262 +243,262 @@ interface TestBinding {
 
   (DOMString or object) receiveUnionIdentity((DOMString or object) arg);
 
-  void passBoolean(boolean arg);
-  void passByte(byte arg);
-  void passOctet(octet arg);
-  void passShort(short arg);
-  void passUnsignedShort(unsigned short arg);
-  void passLong(long arg);
-  void passUnsignedLong(unsigned long arg);
-  void passLongLong(long long arg);
-  void passUnsignedLongLong(unsigned long long arg);
-  void passUnrestrictedFloat(unrestricted float arg);
-  void passFloat(float arg);
-  void passUnrestrictedDouble(unrestricted double arg);
-  void passDouble(double arg);
-  void passString(DOMString arg);
-  void passUsvstring(USVString arg);
-  void passByteString(ByteString arg);
-  void passEnum(TestEnum arg);
-  void passInterface(Blob arg);
-  void passTypedArray(Int8Array arg);
-  void passTypedArray2(ArrayBuffer arg);
-  void passTypedArray3(ArrayBufferView arg);
-  void passUnion((HTMLElement or long) arg);
-  void passUnion2((Event or DOMString) data);
-  void passUnion3((Blob or DOMString) data);
-  void passUnion4((DOMString or sequence<DOMString>) seq);
-  void passUnion5((DOMString or boolean) data);
-  void passUnion6((unsigned long or boolean) bool);
-  void passUnion7((sequence<DOMString> or unsigned long) arg);
-  void passUnion8((sequence<ByteString> or long) arg);
-  void passUnion9((TestDictionary or long) arg);
-  void passUnion10((DOMString or object) arg);
-  void passUnion11((ArrayBuffer or ArrayBufferView) arg);
-  void passUnionWithTypedef((Document or TestTypedef) arg);
-  void passUnionWithTypedef2((sequence<long> or TestTypedef) arg);
-  void passAny(any arg);
-  void passObject(object arg);
-  void passCallbackFunction(Function fun);
-  void passCallbackInterface(EventListener listener);
-  void passSequence(sequence<long> seq);
-  void passAnySequence(sequence<any> seq);
+  undefined passBoolean(boolean arg);
+  undefined passByte(byte arg);
+  undefined passOctet(octet arg);
+  undefined passShort(short arg);
+  undefined passUnsignedShort(unsigned short arg);
+  undefined passLong(long arg);
+  undefined passUnsignedLong(unsigned long arg);
+  undefined passLongLong(long long arg);
+  undefined passUnsignedLongLong(unsigned long long arg);
+  undefined passUnrestrictedFloat(unrestricted float arg);
+  undefined passFloat(float arg);
+  undefined passUnrestrictedDouble(unrestricted double arg);
+  undefined passDouble(double arg);
+  undefined passString(DOMString arg);
+  undefined passUsvstring(USVString arg);
+  undefined passByteString(ByteString arg);
+  undefined passEnum(TestEnum arg);
+  undefined passInterface(Blob arg);
+  undefined passTypedArray(Int8Array arg);
+  undefined passTypedArray2(ArrayBuffer arg);
+  undefined passTypedArray3(ArrayBufferView arg);
+  undefined passUnion((HTMLElement or long) arg);
+  undefined passUnion2((Event or DOMString) data);
+  undefined passUnion3((Blob or DOMString) data);
+  undefined passUnion4((DOMString or sequence<DOMString>) seq);
+  undefined passUnion5((DOMString or boolean) data);
+  undefined passUnion6((unsigned long or boolean) bool);
+  undefined passUnion7((sequence<DOMString> or unsigned long) arg);
+  undefined passUnion8((sequence<ByteString> or long) arg);
+  undefined passUnion9((TestDictionary or long) arg);
+  undefined passUnion10((DOMString or object) arg);
+  undefined passUnion11((ArrayBuffer or ArrayBufferView) arg);
+  undefined passUnionWithTypedef((Document or TestTypedef) arg);
+  undefined passUnionWithTypedef2((sequence<long> or TestTypedef) arg);
+  undefined passAny(any arg);
+  undefined passObject(object arg);
+  undefined passCallbackFunction(Function fun);
+  undefined passCallbackInterface(EventListener listener);
+  undefined passSequence(sequence<long> seq);
+  undefined passAnySequence(sequence<any> seq);
   sequence<any> anySequencePassthrough(sequence<any> seq);
-  void passObjectSequence(sequence<object> seq);
-  void passStringSequence(sequence<DOMString> seq);
-  void passInterfaceSequence(sequence<Blob> seq);
+  undefined passObjectSequence(sequence<object> seq);
+  undefined passStringSequence(sequence<DOMString> seq);
+  undefined passInterfaceSequence(sequence<Blob> seq);
 
-  void passOverloaded(ArrayBuffer arg);
-  void passOverloaded(DOMString arg);
+  undefined passOverloaded(ArrayBuffer arg);
+  undefined passOverloaded(DOMString arg);
 
   // https://github.com/servo/servo/pull/26154
   DOMString passOverloadedDict(Node arg);
   DOMString passOverloadedDict(TestURLLike arg);
 
-  void passNullableBoolean(boolean? arg);
-  void passNullableByte(byte? arg);
-  void passNullableOctet(octet? arg);
-  void passNullableShort(short? arg);
-  void passNullableUnsignedShort(unsigned short? arg);
-  void passNullableLong(long? arg);
-  void passNullableUnsignedLong(unsigned long? arg);
-  void passNullableLongLong(long long? arg);
-  void passNullableUnsignedLongLong(unsigned long long? arg);
-  void passNullableUnrestrictedFloat(unrestricted float? arg);
-  void passNullableFloat(float? arg);
-  void passNullableUnrestrictedDouble(unrestricted double? arg);
-  void passNullableDouble(double? arg);
-  void passNullableString(DOMString? arg);
-  void passNullableUsvstring(USVString? arg);
-  void passNullableByteString(ByteString? arg);
+  undefined passNullableBoolean(boolean? arg);
+  undefined passNullableByte(byte? arg);
+  undefined passNullableOctet(octet? arg);
+  undefined passNullableShort(short? arg);
+  undefined passNullableUnsignedShort(unsigned short? arg);
+  undefined passNullableLong(long? arg);
+  undefined passNullableUnsignedLong(unsigned long? arg);
+  undefined passNullableLongLong(long long? arg);
+  undefined passNullableUnsignedLongLong(unsigned long long? arg);
+  undefined passNullableUnrestrictedFloat(unrestricted float? arg);
+  undefined passNullableFloat(float? arg);
+  undefined passNullableUnrestrictedDouble(unrestricted double? arg);
+  undefined passNullableDouble(double? arg);
+  undefined passNullableString(DOMString? arg);
+  undefined passNullableUsvstring(USVString? arg);
+  undefined passNullableByteString(ByteString? arg);
   // void passNullableEnum(TestEnum? arg);
-  void passNullableInterface(Blob? arg);
-  void passNullableObject(object? arg);
-  void passNullableTypedArray(Int8Array? arg);
-  void passNullableUnion((HTMLElement or long)? arg);
-  void passNullableUnion2((Event or DOMString)? data);
-  void passNullableUnion3((DOMString or sequence<long>)? data);
-  void passNullableUnion4((sequence<long> or boolean)? bool);
-  void passNullableUnion5((unsigned long or boolean)? arg);
-  void passNullableUnion6((ByteString or long)? arg);
-  void passNullableCallbackFunction(Function? fun);
-  void passNullableCallbackInterface(EventListener? listener);
-  void passNullableSequence(sequence<long>? seq);
-
-  void passOptionalBoolean(optional boolean arg);
-  void passOptionalByte(optional byte arg);
-  void passOptionalOctet(optional octet arg);
-  void passOptionalShort(optional short arg);
-  void passOptionalUnsignedShort(optional unsigned short arg);
-  void passOptionalLong(optional long arg);
-  void passOptionalUnsignedLong(optional unsigned long arg);
-  void passOptionalLongLong(optional long long arg);
-  void passOptionalUnsignedLongLong(optional unsigned long long arg);
-  void passOptionalUnrestrictedFloat(optional unrestricted float arg);
-  void passOptionalFloat(optional float arg);
-  void passOptionalUnrestrictedDouble(optional unrestricted double arg);
-  void passOptionalDouble(optional double arg);
-  void passOptionalString(optional DOMString arg);
-  void passOptionalUsvstring(optional USVString arg);
-  void passOptionalByteString(optional ByteString arg);
-  void passOptionalEnum(optional TestEnum arg);
-  void passOptionalInterface(optional Blob arg);
-  void passOptionalUnion(optional (HTMLElement or long) arg);
-  void passOptionalUnion2(optional (Event or DOMString) data);
-  void passOptionalUnion3(optional (DOMString or sequence<long>) arg);
-  void passOptionalUnion4(optional (sequence<long> or boolean) data);
-  void passOptionalUnion5(optional (unsigned long or boolean) bool);
-  void passOptionalUnion6(optional (ByteString or long) arg);
-  void passOptionalAny(optional any arg);
-  void passOptionalObject(optional object arg);
-  void passOptionalCallbackFunction(optional Function fun);
-  void passOptionalCallbackInterface(optional EventListener listener);
-  void passOptionalSequence(optional sequence<long> seq);
-
-  void passOptionalNullableBoolean(optional boolean? arg);
-  void passOptionalNullableByte(optional byte? arg);
-  void passOptionalNullableOctet(optional octet? arg);
-  void passOptionalNullableShort(optional short? arg);
-  void passOptionalNullableUnsignedShort(optional unsigned short? arg);
-  void passOptionalNullableLong(optional long? arg);
-  void passOptionalNullableUnsignedLong(optional unsigned long? arg);
-  void passOptionalNullableLongLong(optional long long? arg);
-  void passOptionalNullableUnsignedLongLong(optional unsigned long long? arg);
-  void passOptionalNullableUnrestrictedFloat(optional unrestricted float? arg);
-  void passOptionalNullableFloat(optional float? arg);
-  void passOptionalNullableUnrestrictedDouble(optional unrestricted double? arg);
-  void passOptionalNullableDouble(optional double? arg);
-  void passOptionalNullableString(optional DOMString? arg);
-  void passOptionalNullableUsvstring(optional USVString? arg);
-  void passOptionalNullableByteString(optional ByteString? arg);
+  undefined passNullableInterface(Blob? arg);
+  undefined passNullableObject(object? arg);
+  undefined passNullableTypedArray(Int8Array? arg);
+  undefined passNullableUnion((HTMLElement or long)? arg);
+  undefined passNullableUnion2((Event or DOMString)? data);
+  undefined passNullableUnion3((DOMString or sequence<long>)? data);
+  undefined passNullableUnion4((sequence<long> or boolean)? bool);
+  undefined passNullableUnion5((unsigned long or boolean)? arg);
+  undefined passNullableUnion6((ByteString or long)? arg);
+  undefined passNullableCallbackFunction(Function? fun);
+  undefined passNullableCallbackInterface(EventListener? listener);
+  undefined passNullableSequence(sequence<long>? seq);
+
+  undefined passOptionalBoolean(optional boolean arg);
+  undefined passOptionalByte(optional byte arg);
+  undefined passOptionalOctet(optional octet arg);
+  undefined passOptionalShort(optional short arg);
+  undefined passOptionalUnsignedShort(optional unsigned short arg);
+  undefined passOptionalLong(optional long arg);
+  undefined passOptionalUnsignedLong(optional unsigned long arg);
+  undefined passOptionalLongLong(optional long long arg);
+  undefined passOptionalUnsignedLongLong(optional unsigned long long arg);
+  undefined passOptionalUnrestrictedFloat(optional unrestricted float arg);
+  undefined passOptionalFloat(optional float arg);
+  undefined passOptionalUnrestrictedDouble(optional unrestricted double arg);
+  undefined passOptionalDouble(optional double arg);
+  undefined passOptionalString(optional DOMString arg);
+  undefined passOptionalUsvstring(optional USVString arg);
+  undefined passOptionalByteString(optional ByteString arg);
+  undefined passOptionalEnum(optional TestEnum arg);
+  undefined passOptionalInterface(optional Blob arg);
+  undefined passOptionalUnion(optional (HTMLElement or long) arg);
+  undefined passOptionalUnion2(optional (Event or DOMString) data);
+  undefined passOptionalUnion3(optional (DOMString or sequence<long>) arg);
+  undefined passOptionalUnion4(optional (sequence<long> or boolean) data);
+  undefined passOptionalUnion5(optional (unsigned long or boolean) bool);
+  undefined passOptionalUnion6(optional (ByteString or long) arg);
+  undefined passOptionalAny(optional any arg);
+  undefined passOptionalObject(optional object arg);
+  undefined passOptionalCallbackFunction(optional Function fun);
+  undefined passOptionalCallbackInterface(optional EventListener listener);
+  undefined passOptionalSequence(optional sequence<long> seq);
+
+  undefined passOptionalNullableBoolean(optional boolean? arg);
+  undefined passOptionalNullableByte(optional byte? arg);
+  undefined passOptionalNullableOctet(optional octet? arg);
+  undefined passOptionalNullableShort(optional short? arg);
+  undefined passOptionalNullableUnsignedShort(optional unsigned short? arg);
+  undefined passOptionalNullableLong(optional long? arg);
+  undefined passOptionalNullableUnsignedLong(optional unsigned long? arg);
+  undefined passOptionalNullableLongLong(optional long long? arg);
+  undefined passOptionalNullableUnsignedLongLong(optional unsigned long long? arg);
+  undefined passOptionalNullableUnrestrictedFloat(optional unrestricted float? arg);
+  undefined passOptionalNullableFloat(optional float? arg);
+  undefined passOptionalNullableUnrestrictedDouble(optional unrestricted double? arg);
+  undefined passOptionalNullableDouble(optional double? arg);
+  undefined passOptionalNullableString(optional DOMString? arg);
+  undefined passOptionalNullableUsvstring(optional USVString? arg);
+  undefined passOptionalNullableByteString(optional ByteString? arg);
   // void passOptionalNullableEnum(optional TestEnum? arg);
-  void passOptionalNullableInterface(optional Blob? arg);
-  void passOptionalNullableObject(optional object? arg);
-  void passOptionalNullableUnion(optional (HTMLElement or long)? arg);
-  void passOptionalNullableUnion2(optional (Event or DOMString)? data);
-  void passOptionalNullableUnion3(optional (DOMString or sequence<long>)? arg);
-  void passOptionalNullableUnion4(optional (sequence<long> or boolean)? data);
-  void passOptionalNullableUnion5(optional (unsigned long or boolean)? bool);
-  void passOptionalNullableUnion6(optional (ByteString or long)? arg);
-  void passOptionalNullableCallbackFunction(optional Function? fun);
-  void passOptionalNullableCallbackInterface(optional EventListener? listener);
-  void passOptionalNullableSequence(optional sequence<long>? seq);
-
-  void passOptionalBooleanWithDefault(optional boolean arg = false);
-  void passOptionalByteWithDefault(optional byte arg = 0);
-  void passOptionalOctetWithDefault(optional octet arg = 19);
-  void passOptionalShortWithDefault(optional short arg = 5);
-  void passOptionalUnsignedShortWithDefault(optional unsigned short arg = 2);
-  void passOptionalLongWithDefault(optional long arg = 7);
-  void passOptionalUnsignedLongWithDefault(optional unsigned long arg = 6);
-  void passOptionalLongLongWithDefault(optional long long arg = -12);
-  void passOptionalUnsignedLongLongWithDefault(optional unsigned long long arg = 17);
-  void passOptionalBytestringWithDefault(optional ByteString arg = "x");
-  void passOptionalStringWithDefault(optional DOMString arg = "x");
-  void passOptionalUsvstringWithDefault(optional USVString arg = "x");
-  void passOptionalEnumWithDefault(optional TestEnum arg = "foo");
-  void passOptionalSequenceWithDefault(optional sequence<long> seq = []);
+  undefined passOptionalNullableInterface(optional Blob? arg);
+  undefined passOptionalNullableObject(optional object? arg);
+  undefined passOptionalNullableUnion(optional (HTMLElement or long)? arg);
+  undefined passOptionalNullableUnion2(optional (Event or DOMString)? data);
+  undefined passOptionalNullableUnion3(optional (DOMString or sequence<long>)? arg);
+  undefined passOptionalNullableUnion4(optional (sequence<long> or boolean)? data);
+  undefined passOptionalNullableUnion5(optional (unsigned long or boolean)? bool);
+  undefined passOptionalNullableUnion6(optional (ByteString or long)? arg);
+  undefined passOptionalNullableCallbackFunction(optional Function? fun);
+  undefined passOptionalNullableCallbackInterface(optional EventListener? listener);
+  undefined passOptionalNullableSequence(optional sequence<long>? seq);
+
+  undefined passOptionalBooleanWithDefault(optional boolean arg = false);
+  undefined passOptionalByteWithDefault(optional byte arg = 0);
+  undefined passOptionalOctetWithDefault(optional octet arg = 19);
+  undefined passOptionalShortWithDefault(optional short arg = 5);
+  undefined passOptionalUnsignedShortWithDefault(optional unsigned short arg = 2);
+  undefined passOptionalLongWithDefault(optional long arg = 7);
+  undefined passOptionalUnsignedLongWithDefault(optional unsigned long arg = 6);
+  undefined passOptionalLongLongWithDefault(optional long long arg = -12);
+  undefined passOptionalUnsignedLongLongWithDefault(optional unsigned long long arg = 17);
+  undefined passOptionalBytestringWithDefault(optional ByteString arg = "x");
+  undefined passOptionalStringWithDefault(optional DOMString arg = "x");
+  undefined passOptionalUsvstringWithDefault(optional USVString arg = "x");
+  undefined passOptionalEnumWithDefault(optional TestEnum arg = "foo");
+  undefined passOptionalSequenceWithDefault(optional sequence<long> seq = []);
   // void passOptionalUnionWithDefault(optional (HTMLElement or long) arg = 9);
   // void passOptionalUnion2WithDefault(optional(Event or DOMString)? data = "foo");
 
-  void passOptionalNullableBooleanWithDefault(optional boolean? arg = null);
-  void passOptionalNullableByteWithDefault(optional byte? arg = null);
-  void passOptionalNullableOctetWithDefault(optional octet? arg = null);
-  void passOptionalNullableShortWithDefault(optional short? arg = null);
-  void passOptionalNullableUnsignedShortWithDefault(optional unsigned short? arg = null);
-  void passOptionalNullableLongWithDefault(optional long? arg = null);
-  void passOptionalNullableUnsignedLongWithDefault(optional unsigned long? arg = null);
-  void passOptionalNullableLongLongWithDefault(optional long long? arg = null);
-  void passOptionalNullableUnsignedLongLongWithDefault(optional unsigned long long? arg = null);
-  void passOptionalNullableStringWithDefault(optional DOMString? arg = null);
-  void passOptionalNullableUsvstringWithDefault(optional USVString? arg = null);
-  void passOptionalNullableByteStringWithDefault(optional ByteString? arg = null);
+  undefined passOptionalNullableBooleanWithDefault(optional boolean? arg = null);
+  undefined passOptionalNullableByteWithDefault(optional byte? arg = null);
+  undefined passOptionalNullableOctetWithDefault(optional octet? arg = null);
+  undefined passOptionalNullableShortWithDefault(optional short? arg = null);
+  undefined passOptionalNullableUnsignedShortWithDefault(optional unsigned short? arg = null);
+  undefined passOptionalNullableLongWithDefault(optional long? arg = null);
+  undefined passOptionalNullableUnsignedLongWithDefault(optional unsigned long? arg = null);
+  undefined passOptionalNullableLongLongWithDefault(optional long long? arg = null);
+  undefined passOptionalNullableUnsignedLongLongWithDefault(optional unsigned long long? arg = null);
+  undefined passOptionalNullableStringWithDefault(optional DOMString? arg = null);
+  undefined passOptionalNullableUsvstringWithDefault(optional USVString? arg = null);
+  undefined passOptionalNullableByteStringWithDefault(optional ByteString? arg = null);
   // void passOptionalNullableEnumWithDefault(optional TestEnum? arg = null);
-  void passOptionalNullableInterfaceWithDefault(optional Blob? arg = null);
-  void passOptionalNullableObjectWithDefault(optional object? arg = null);
-  void passOptionalNullableUnionWithDefault(optional (HTMLElement or long)? arg = null);
-  void passOptionalNullableUnion2WithDefault(optional (Event or DOMString)? data = null);
+  undefined passOptionalNullableInterfaceWithDefault(optional Blob? arg = null);
+  undefined passOptionalNullableObjectWithDefault(optional object? arg = null);
+  undefined passOptionalNullableUnionWithDefault(optional (HTMLElement or long)? arg = null);
+  undefined passOptionalNullableUnion2WithDefault(optional (Event or DOMString)? data = null);
   // void passOptionalNullableCallbackFunctionWithDefault(optional Function? fun = null);
-  void passOptionalNullableCallbackInterfaceWithDefault(optional EventListener? listener = null);
-  void passOptionalAnyWithDefault(optional any arg = null);
-
-  void passOptionalNullableBooleanWithNonNullDefault(optional boolean? arg = false);
-  void passOptionalNullableByteWithNonNullDefault(optional byte? arg = 7);
-  void passOptionalNullableOctetWithNonNullDefault(optional octet? arg = 7);
-  void passOptionalNullableShortWithNonNullDefault(optional short? arg = 7);
-  void passOptionalNullableUnsignedShortWithNonNullDefault(optional unsigned short? arg = 7);
-  void passOptionalNullableLongWithNonNullDefault(optional long? arg = 7);
-  void passOptionalNullableUnsignedLongWithNonNullDefault(optional unsigned long? arg = 7);
-  void passOptionalNullableLongLongWithNonNullDefault(optional long long? arg = 7);
-  void passOptionalNullableUnsignedLongLongWithNonNullDefault(optional unsigned long long? arg = 7);
+  undefined passOptionalNullableCallbackInterfaceWithDefault(optional EventListener? listener = null);
+  undefined passOptionalAnyWithDefault(optional any arg = null);
+
+  undefined passOptionalNullableBooleanWithNonNullDefault(optional boolean? arg = false);
+  undefined passOptionalNullableByteWithNonNullDefault(optional byte? arg = 7);
+  undefined passOptionalNullableOctetWithNonNullDefault(optional octet? arg = 7);
+  undefined passOptionalNullableShortWithNonNullDefault(optional short? arg = 7);
+  undefined passOptionalNullableUnsignedShortWithNonNullDefault(optional unsigned short? arg = 7);
+  undefined passOptionalNullableLongWithNonNullDefault(optional long? arg = 7);
+  undefined passOptionalNullableUnsignedLongWithNonNullDefault(optional unsigned long? arg = 7);
+  undefined passOptionalNullableLongLongWithNonNullDefault(optional long long? arg = 7);
+  undefined passOptionalNullableUnsignedLongLongWithNonNullDefault(optional unsigned long long? arg = 7);
   // void passOptionalNullableUnrestrictedFloatWithNonNullDefault(optional unrestricted float? arg = 0.0);
   // void passOptionalNullableFloatWithNonNullDefault(optional float? arg = 0.0);
   // void passOptionalNullableUnrestrictedDoubleWithNonNullDefault(optional unrestricted double? arg = 0.0);
   // void passOptionalNullableDoubleWithNonNullDefault(optional double? arg = 0.0);
-  void passOptionalNullableStringWithNonNullDefault(optional DOMString? arg = "x");
-  void passOptionalNullableUsvstringWithNonNullDefault(optional USVString? arg = "x");
+  undefined passOptionalNullableStringWithNonNullDefault(optional DOMString? arg = "x");
+  undefined passOptionalNullableUsvstringWithNonNullDefault(optional USVString? arg = "x");
   // void passOptionalNullableEnumWithNonNullDefault(optional TestEnum? arg = "foo");
   // void passOptionalNullableUnionWithNonNullDefault(optional (HTMLElement or long)? arg = 7);
   // void passOptionalNullableUnion2WithNonNullDefault(optional (Event or DOMString)? data = "foo");
   TestBinding passOptionalOverloaded(TestBinding arg0, optional unsigned long arg1 = 0,
                                      optional unsigned long arg2 = 0);
-  void passOptionalOverloaded(Blob arg0, optional unsigned long arg1 = 0);
-
-  void passVariadicBoolean(boolean... args);
-  void passVariadicBooleanAndDefault(optional boolean arg = true, boolean... args);
-  void passVariadicByte(byte... args);
-  void passVariadicOctet(octet... args);
-  void passVariadicShort(short... args);
-  void passVariadicUnsignedShort(unsigned short... args);
-  void passVariadicLong(long... args);
-  void passVariadicUnsignedLong(unsigned long... args);
-  void passVariadicLongLong(long long... args);
-  void passVariadicUnsignedLongLong(unsigned long long... args);
-  void passVariadicUnrestrictedFloat(unrestricted float... args);
-  void passVariadicFloat(float... args);
-  void passVariadicUnrestrictedDouble(unrestricted double... args);
-  void passVariadicDouble(double... args);
-  void passVariadicString(DOMString... args);
-  void passVariadicUsvstring(USVString... args);
-  void passVariadicByteString(ByteString... args);
-  void passVariadicEnum(TestEnum... args);
-  void passVariadicInterface(Blob... args);
-  void passVariadicUnion((HTMLElement or long)... args);
-  void passVariadicUnion2((Event or DOMString)... args);
-  void passVariadicUnion3((Blob or DOMString)... args);
-  void passVariadicUnion4((Blob or boolean)... args);
-  void passVariadicUnion5((DOMString or unsigned long)... args);
-  void passVariadicUnion6((unsigned long or boolean)... args);
-  void passVariadicUnion7((ByteString or long)... args);
-  void passVariadicAny(any... args);
-  void passVariadicObject(object... args);
-
-  void passSequenceSequence(sequence<sequence<long>> seq);
+  undefined passOptionalOverloaded(Blob arg0, optional unsigned long arg1 = 0);
+
+  undefined passVariadicBoolean(boolean... args);
+  undefined passVariadicBooleanAndDefault(optional boolean arg = true, boolean... args);
+  undefined passVariadicByte(byte... args);
+  undefined passVariadicOctet(octet... args);
+  undefined passVariadicShort(short... args);
+  undefined passVariadicUnsignedShort(unsigned short... args);
+  undefined passVariadicLong(long... args);
+  undefined passVariadicUnsignedLong(unsigned long... args);
+  undefined passVariadicLongLong(long long... args);
+  undefined passVariadicUnsignedLongLong(unsigned long long... args);
+  undefined passVariadicUnrestrictedFloat(unrestricted float... args);
+  undefined passVariadicFloat(float... args);
+  undefined passVariadicUnrestrictedDouble(unrestricted double... args);
+  undefined passVariadicDouble(double... args);
+  undefined passVariadicString(DOMString... args);
+  undefined passVariadicUsvstring(USVString... args);
+  undefined passVariadicByteString(ByteString... args);
+  undefined passVariadicEnum(TestEnum... args);
+  undefined passVariadicInterface(Blob... args);
+  undefined passVariadicUnion((HTMLElement or long)... args);
+  undefined passVariadicUnion2((Event or DOMString)... args);
+  undefined passVariadicUnion3((Blob or DOMString)... args);
+  undefined passVariadicUnion4((Blob or boolean)... args);
+  undefined passVariadicUnion5((DOMString or unsigned long)... args);
+  undefined passVariadicUnion6((unsigned long or boolean)... args);
+  undefined passVariadicUnion7((ByteString or long)... args);
+  undefined passVariadicAny(any... args);
+  undefined passVariadicObject(object... args);
+
+  undefined passSequenceSequence(sequence<sequence<long>> seq);
   sequence<sequence<long>> returnSequenceSequence();
-  void passUnionSequenceSequence((long or sequence<sequence<long>>) seq);
-
-  void passRecord(record<DOMString, long> arg);
-  void passRecordWithUSVStringKey(record<USVString, long> arg);
-  void passRecordWithByteStringKey(record<ByteString, long> arg);
-  void passNullableRecord(record<DOMString, long>? arg);
-  void passRecordOfNullableInts(record<DOMString, long?> arg);
-  void passOptionalRecordOfNullableInts(optional record<DOMString, long?> arg);
-  void passOptionalNullableRecordOfNullableInts(optional record<DOMString, long?>? arg);
-  void passCastableObjectRecord(record<DOMString, TestBinding> arg);
-  void passNullableCastableObjectRecord(record<DOMString, TestBinding?> arg);
-  void passCastableObjectNullableRecord(record<DOMString, TestBinding>? arg);
-  void passNullableCastableObjectNullableRecord(record<DOMString, TestBinding?>? arg);
-  void passOptionalRecord(optional record<DOMString, long> arg);
-  void passOptionalNullableRecord(optional record<DOMString, long>? arg);
-  void passOptionalNullableRecordWithDefaultValue(optional record<DOMString, long>? arg = null);
-  void passOptionalObjectRecord(optional record<DOMString, TestBinding> arg);
-  void passStringRecord(record<DOMString, DOMString> arg);
-  void passByteStringRecord(record<DOMString, ByteString> arg);
-  void passRecordOfRecords(record<DOMString, record<DOMString, long>> arg);
-
-  void passRecordUnion((long or record<DOMString, ByteString>) init);
-  void passRecordUnion2((TestBinding or record<DOMString, ByteString>) init);
-  void passRecordUnion3((TestBinding or sequence<sequence<ByteString>> or record<DOMString, ByteString>) init);
+  undefined passUnionSequenceSequence((long or sequence<sequence<long>>) seq);
+
+  undefined passRecord(record<DOMString, long> arg);
+  undefined passRecordWithUSVStringKey(record<USVString, long> arg);
+  undefined passRecordWithByteStringKey(record<ByteString, long> arg);
+  undefined passNullableRecord(record<DOMString, long>? arg);
+  undefined passRecordOfNullableInts(record<DOMString, long?> arg);
+  undefined passOptionalRecordOfNullableInts(optional record<DOMString, long?> arg);
+  undefined passOptionalNullableRecordOfNullableInts(optional record<DOMString, long?>? arg);
+  undefined passCastableObjectRecord(record<DOMString, TestBinding> arg);
+  undefined passNullableCastableObjectRecord(record<DOMString, TestBinding?> arg);
+  undefined passCastableObjectNullableRecord(record<DOMString, TestBinding>? arg);
+  undefined passNullableCastableObjectNullableRecord(record<DOMString, TestBinding?>? arg);
+  undefined passOptionalRecord(optional record<DOMString, long> arg);
+  undefined passOptionalNullableRecord(optional record<DOMString, long>? arg);
+  undefined passOptionalNullableRecordWithDefaultValue(optional record<DOMString, long>? arg = null);
+  undefined passOptionalObjectRecord(optional record<DOMString, TestBinding> arg);
+  undefined passStringRecord(record<DOMString, DOMString> arg);
+  undefined passByteStringRecord(record<DOMString, ByteString> arg);
+  undefined passRecordOfRecords(record<DOMString, record<DOMString, long>> arg);
+
+  undefined passRecordUnion((long or record<DOMString, ByteString>) init);
+  undefined passRecordUnion2((TestBinding or record<DOMString, ByteString>) init);
+  undefined passRecordUnion3((TestBinding or sequence<sequence<ByteString>> or record<DOMString, ByteString>) init);
 
   record<DOMString, long> receiveRecord();
   record<USVString, long> receiveRecordWithUSVStringKey();
@@ -510,7 +510,7 @@ interface TestBinding {
   record<DOMString, any> receiveAnyRecord();
 
   static attribute boolean booleanAttributeStatic;
-  static void receiveVoidStatic();
+  static undefined receiveVoidStatic();
   boolean BooleanMozPreference(DOMString pref_name);
   DOMString StringMozPreference(DOMString pref_name);
 
@@ -519,22 +519,22 @@ interface TestBinding {
   [Pref="dom.testbinding.prefcontrolled.enabled"]
   static readonly attribute boolean prefControlledStaticAttributeDisabled;
   [Pref="dom.testbinding.prefcontrolled.enabled"]
-  void prefControlledMethodDisabled();
+  undefined prefControlledMethodDisabled();
   [Pref="dom.testbinding.prefcontrolled.enabled"]
-  static void prefControlledStaticMethodDisabled();
+  static undefined prefControlledStaticMethodDisabled();
   [Pref="dom.testbinding.prefcontrolled.enabled"]
   const unsigned short prefControlledConstDisabled = 0;
   [Pref="layout.animations.test.enabled"]
-  void advanceClock(long millis);
+  undefined advanceClock(long millis);
 
   [Pref="dom.testbinding.prefcontrolled2.enabled"]
   readonly attribute boolean prefControlledAttributeEnabled;
   [Pref="dom.testbinding.prefcontrolled2.enabled"]
   static readonly attribute boolean prefControlledStaticAttributeEnabled;
   [Pref="dom.testbinding.prefcontrolled2.enabled"]
-  void prefControlledMethodEnabled();
+  undefined prefControlledMethodEnabled();
   [Pref="dom.testbinding.prefcontrolled2.enabled"]
-  static void prefControlledStaticMethodEnabled();
+  static undefined prefControlledStaticMethodEnabled();
   [Pref="dom.testbinding.prefcontrolled2.enabled"]
   const unsigned short prefControlledConstEnabled = 0;
 
@@ -543,9 +543,9 @@ interface TestBinding {
   [Func="TestBinding::condition_unsatisfied"]
   static readonly attribute boolean funcControlledStaticAttributeDisabled;
   [Func="TestBinding::condition_unsatisfied"]
-  void funcControlledMethodDisabled();
+  undefined funcControlledMethodDisabled();
   [Func="TestBinding::condition_unsatisfied"]
-  static void funcControlledStaticMethodDisabled();
+  static undefined funcControlledStaticMethodDisabled();
   [Func="TestBinding::condition_unsatisfied"]
   const unsigned short funcControlledConstDisabled = 0;
 
@@ -554,9 +554,9 @@ interface TestBinding {
   [Func="TestBinding::condition_satisfied"]
   static readonly attribute boolean funcControlledStaticAttributeEnabled;
   [Func="TestBinding::condition_satisfied"]
-  void funcControlledMethodEnabled();
+  undefined funcControlledMethodEnabled();
   [Func="TestBinding::condition_satisfied"]
-  static void funcControlledStaticMethodEnabled();
+  static undefined funcControlledStaticMethodEnabled();
   [Func="TestBinding::condition_satisfied"]
   const unsigned short funcControlledConstEnabled = 0;
 
@@ -565,14 +565,14 @@ interface TestBinding {
   [Throws]
   Promise<any> returnRejectedPromise(any value);
   readonly attribute Promise<boolean> promiseAttribute;
-  void acceptPromise(Promise<DOMString> string);
+  undefined acceptPromise(Promise<DOMString> string);
   Promise<any> promiseNativeHandler(SimpleCallback? resolve, SimpleCallback? reject);
-  void promiseResolveNative(Promise<any> p, any value);
-  void promiseRejectNative(Promise<any> p, any value);
-  void promiseRejectWithTypeError(Promise<any> p, USVString message);
-  void resolvePromiseDelayed(Promise<any> p, DOMString value, unsigned long long ms);
+  undefined promiseResolveNative(Promise<any> p, any value);
+  undefined promiseRejectNative(Promise<any> p, any value);
+  undefined promiseRejectWithTypeError(Promise<any> p, USVString message);
+  undefined resolvePromiseDelayed(Promise<any> p, DOMString value, unsigned long long ms);
 
-  void panic();
+  undefined panic();
 
   GlobalScope entryGlobal();
   GlobalScope incumbentGlobal();
@@ -593,9 +593,9 @@ partial interface TestBinding {
   readonly attribute boolean semiExposedBoolFromPartialInterface;
 };
 
-callback SimpleCallback = void(any value);
+callback SimpleCallback = undefined(any value);
 
 partial interface TestBinding {
   [Pref="dom.testable_crash.enabled"]
-  void crashHard();
+  undefined crashHard();
 };
diff --git a/components/script/dom/webidls/TestBindingIterable.webidl b/components/script/dom/webidls/TestBindingIterable.webidl
index d74c43d8f5f..46a2d319e3d 100644
--- a/components/script/dom/webidls/TestBindingIterable.webidl
+++ b/components/script/dom/webidls/TestBindingIterable.webidl
@@ -8,7 +8,7 @@
 [Pref="dom.testbinding.enabled", Exposed=(Window,Worker)]
 interface TestBindingIterable {
   [Throws] constructor();
-  void add(DOMString arg);
+  undefined add(DOMString arg);
   readonly attribute unsigned long length;
   getter DOMString getItem(unsigned long index);
   iterable<DOMString>;
diff --git a/components/script/dom/webidls/TestBindingPairIterable.webidl b/components/script/dom/webidls/TestBindingPairIterable.webidl
index 5d19fbe9f32..64499e7464c 100644
--- a/components/script/dom/webidls/TestBindingPairIterable.webidl
+++ b/components/script/dom/webidls/TestBindingPairIterable.webidl
@@ -8,6 +8,6 @@
 [Pref="dom.testbinding.enabled", Exposed=(Window,Worker)]
 interface TestBindingPairIterable {
   [Throws] constructor();
-  void add(DOMString key, unsigned long value);
+  undefined add(DOMString key, unsigned long value);
   iterable<DOMString, unsigned long>;
 };
diff --git a/components/script/dom/webidls/TestBindingProxy.webidl b/components/script/dom/webidls/TestBindingProxy.webidl
index 211a4f3a9f7..b16d5104242 100644
--- a/components/script/dom/webidls/TestBindingProxy.webidl
+++ b/components/script/dom/webidls/TestBindingProxy.webidl
@@ -17,13 +17,13 @@ interface TestBindingProxy : TestBinding {
 
   getter DOMString getNamedItem(DOMString item_name);
 
-  setter void setNamedItem(DOMString item_name, DOMString value);
+  setter undefined setNamedItem(DOMString item_name, DOMString value);
 
   getter DOMString getItem(unsigned long index);
 
-  setter void setItem(unsigned long index, DOMString value);
+  setter undefined setItem(unsigned long index, DOMString value);
 
-  deleter void removeItem(DOMString name);
+  deleter undefined removeItem(DOMString name);
 
   stringifier;
 };
diff --git a/components/script/dom/webidls/TestRunner.webidl b/components/script/dom/webidls/TestRunner.webidl
index bea35374bd6..448be17c4d4 100644
--- a/components/script/dom/webidls/TestRunner.webidl
+++ b/components/script/dom/webidls/TestRunner.webidl
@@ -9,7 +9,7 @@
 [Pref="dom.bluetooth.testing.enabled", Exposed=Window]
 interface TestRunner {
   [Throws]
-  void setBluetoothMockDataSet(DOMString dataSetName);
+  undefined setBluetoothMockDataSet(DOMString dataSetName);
   // void setBluetoothManualChooser();
   // void getBluetoothManualChooserEvents(BluetoothManualChooserEventsCallback callback);
   // void sendBluetoothManualChooserEvent(DOMString event, DOMString argument);
diff --git a/components/script/dom/webidls/TestWorklet.webidl b/components/script/dom/webidls/TestWorklet.webidl
index be254a3d079..81b02da0731 100644
--- a/components/script/dom/webidls/TestWorklet.webidl
+++ b/components/script/dom/webidls/TestWorklet.webidl
@@ -8,6 +8,6 @@
 [Pref="dom.worklet.testing.enabled", Exposed=(Window)]
 interface TestWorklet {
    [Throws] constructor();
-   [NewObject] Promise<void> addModule(USVString moduleURL, optional WorkletOptions options = {});
+   [NewObject] Promise<undefined> addModule(USVString moduleURL, optional WorkletOptions options = {});
    DOMString? lookup(DOMString key);
 };
diff --git a/components/script/dom/webidls/TestWorkletGlobalScope.webidl b/components/script/dom/webidls/TestWorkletGlobalScope.webidl
index e3cbec7c052..1c2735dbc65 100644
--- a/components/script/dom/webidls/TestWorkletGlobalScope.webidl
+++ b/components/script/dom/webidls/TestWorkletGlobalScope.webidl
@@ -7,5 +7,5 @@
 
 [Global=(Worklet,TestWorklet), Pref="dom.worklet.enabled", Exposed=TestWorklet]
 interface TestWorkletGlobalScope : WorkletGlobalScope {
-    void registerKeyValue(DOMString key, DOMString value);
+    undefined registerKeyValue(DOMString key, DOMString value);
 };
diff --git a/components/script/dom/webidls/TextTrack.webidl b/components/script/dom/webidls/TextTrack.webidl
index d51d9b0dc67..c0016ca6ed9 100644
--- a/components/script/dom/webidls/TextTrack.webidl
+++ b/components/script/dom/webidls/TextTrack.webidl
@@ -22,9 +22,9 @@ interface TextTrack : EventTarget {
   readonly attribute TextTrackCueList? activeCues;
 
   [Throws]
-  void addCue(TextTrackCue cue);
+  undefined addCue(TextTrackCue cue);
   [Throws]
-  void removeCue(TextTrackCue cue);
+  undefined removeCue(TextTrackCue cue);
 
   attribute EventHandler oncuechange;
 };
diff --git a/components/script/dom/webidls/UIEvent.webidl b/components/script/dom/webidls/UIEvent.webidl
index 12850f70e45..04eb12c2083 100644
--- a/components/script/dom/webidls/UIEvent.webidl
+++ b/components/script/dom/webidls/UIEvent.webidl
@@ -21,5 +21,11 @@ dictionary UIEventInit : EventInit {
 // https://w3c.github.io/uievents/#idl-interface-UIEvent-initializers
 partial interface UIEvent {
     // Deprecated in DOM Level 3
-    void initUIEvent (DOMString typeArg, boolean bubblesArg, boolean cancelableArg, Window? viewArg, long detailArg);
+    undefined initUIEvent (
+      DOMString typeArg,
+      boolean bubblesArg,
+      boolean cancelableArg,
+      Window? viewArg,
+      long detailArg
+    );
 };
diff --git a/components/script/dom/webidls/URL.webidl b/components/script/dom/webidls/URL.webidl
index 47a1ef53129..7059fe43ea5 100644
--- a/components/script/dom/webidls/URL.webidl
+++ b/components/script/dom/webidls/URL.webidl
@@ -24,7 +24,7 @@ interface URL {
   // https://w3c.github.io/FileAPI/#creating-revoking
   static DOMString createObjectURL(Blob blob);
   // static DOMString createFor(Blob blob);
-  static void revokeObjectURL(DOMString url);
+  static undefined revokeObjectURL(DOMString url);
 
   USVString toJSON();
 };
diff --git a/components/script/dom/webidls/URLSearchParams.webidl b/components/script/dom/webidls/URLSearchParams.webidl
index 33ba9253624..8d1d2a48392 100644
--- a/components/script/dom/webidls/URLSearchParams.webidl
+++ b/components/script/dom/webidls/URLSearchParams.webidl
@@ -9,14 +9,14 @@
 [Exposed=(Window,Worker)]
 interface URLSearchParams {
   [Throws] constructor(optional (sequence<sequence<USVString>> or record<USVString, USVString> or USVString) init = "");
-  void append(USVString name, USVString value);
-  void delete(USVString name);
+  undefined append(USVString name, USVString value);
+  undefined delete(USVString name);
   USVString? get(USVString name);
   sequence<USVString> getAll(USVString name);
   boolean has(USVString name);
-  void set(USVString name, USVString value);
+  undefined set(USVString name, USVString value);
 
-  void sort();
+  undefined sort();
 
   // Be careful with implementing iterable interface.
   // Search params might be mutated by URL::SetSearch while iterating (discussed in PR #10351).
diff --git a/components/script/dom/webidls/VoidFunction.webidl b/components/script/dom/webidls/VoidFunction.webidl
index 6c219ff9380..551a69d87e0 100644
--- a/components/script/dom/webidls/VoidFunction.webidl
+++ b/components/script/dom/webidls/VoidFunction.webidl
@@ -10,4 +10,4 @@
  * and create derivative works of this document.
  */
 
-callback VoidFunction = void ();
+callback VoidFunction = undefined ();
diff --git a/components/script/dom/webidls/WebGL2RenderingContext.webidl b/components/script/dom/webidls/WebGL2RenderingContext.webidl
index 632f1ce5cc6..3d5d086f554 100644
--- a/components/script/dom/webidls/WebGL2RenderingContext.webidl
+++ b/components/script/dom/webidls/WebGL2RenderingContext.webidl
@@ -286,33 +286,33 @@ interface mixin WebGL2RenderingContextBase
   const GLenum MAX_CLIENT_WAIT_TIMEOUT_WEBGL                 = 0x9247;
 
   /* Buffer objects */
-  void copyBufferSubData(GLenum readTarget, GLenum writeTarget, GLintptr readOffset,
+  undefined copyBufferSubData(GLenum readTarget, GLenum writeTarget, GLintptr readOffset,
                          GLintptr writeOffset, GLsizeiptr size);
   // MapBufferRange, in particular its read-only and write-only modes,
   // can not be exposed safely to JavaScript. GetBufferSubData
   // replaces it for the purpose of fetching data back from the GPU.
-  void getBufferSubData(GLenum target, GLintptr srcByteOffset, /*[AllowShared]*/ ArrayBufferView dstBuffer,
+  undefined getBufferSubData(GLenum target, GLintptr srcByteOffset, /*[AllowShared]*/ ArrayBufferView dstBuffer,
                         optional GLuint dstOffset = 0, optional GLuint length = 0);
 
   /* Framebuffer objects */
   // void blitFramebuffer(GLint srcX0, GLint srcY0, GLint srcX1, GLint srcY1, GLint dstX0, GLint dstY0,
   //                      GLint dstX1, GLint dstY1, GLbitfield mask, GLenum filter);
-  void framebufferTextureLayer(GLenum target, GLenum attachment, WebGLTexture? texture, GLint level,
+  undefined framebufferTextureLayer(GLenum target, GLenum attachment, WebGLTexture? texture, GLint level,
                                GLint layer);
-  void invalidateFramebuffer(GLenum target, sequence<GLenum> attachments);
-  void invalidateSubFramebuffer(GLenum target, sequence<GLenum> attachments,
+  undefined invalidateFramebuffer(GLenum target, sequence<GLenum> attachments);
+  undefined invalidateSubFramebuffer(GLenum target, sequence<GLenum> attachments,
                                 GLint x, GLint y, GLsizei width, GLsizei height);
-  void readBuffer(GLenum src);
+  undefined readBuffer(GLenum src);
 
   /* Renderbuffer objects */
   any getInternalformatParameter(GLenum target, GLenum internalformat, GLenum pname);
-  void renderbufferStorageMultisample(GLenum target, GLsizei samples, GLenum internalformat,
+  undefined renderbufferStorageMultisample(GLenum target, GLsizei samples, GLenum internalformat,
                                       GLsizei width, GLsizei height);
 
   /* Texture objects */
-  void texStorage2D(GLenum target, GLsizei levels, GLenum internalformat, GLsizei width,
+  undefined texStorage2D(GLenum target, GLsizei levels, GLenum internalformat, GLsizei width,
                     GLsizei height);
-  void texStorage3D(GLenum target, GLsizei levels, GLenum internalformat, GLsizei width,
+  undefined texStorage3D(GLenum target, GLsizei levels, GLenum internalformat, GLsizei width,
                     GLsizei height, GLsizei depth);
 
   //[Throws]
@@ -365,155 +365,155 @@ interface mixin WebGL2RenderingContextBase
   [WebGLHandlesContextLoss] GLint getFragDataLocation(WebGLProgram program, DOMString name);
 
   /* Uniforms */
-  void uniform1ui(WebGLUniformLocation? location, GLuint v0);
-  void uniform2ui(WebGLUniformLocation? location, GLuint v0, GLuint v1);
-  void uniform3ui(WebGLUniformLocation? location, GLuint v0, GLuint v1, GLuint v2);
-  void uniform4ui(WebGLUniformLocation? location, GLuint v0, GLuint v1, GLuint v2, GLuint v3);
+  undefined uniform1ui(WebGLUniformLocation? location, GLuint v0);
+  undefined uniform2ui(WebGLUniformLocation? location, GLuint v0, GLuint v1);
+  undefined uniform3ui(WebGLUniformLocation? location, GLuint v0, GLuint v1, GLuint v2);
+  undefined uniform4ui(WebGLUniformLocation? location, GLuint v0, GLuint v1, GLuint v2, GLuint v3);
 
-  void uniform1uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
+  undefined uniform1uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
                    optional GLuint srcLength = 0);
-  void uniform2uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
+  undefined uniform2uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
                    optional GLuint srcLength = 0);
-  void uniform3uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
+  undefined uniform3uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
                    optional GLuint srcLength = 0);
-  void uniform4uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
+  undefined uniform4uiv(WebGLUniformLocation? location, Uint32List data, optional GLuint srcOffset = 0,
                    optional GLuint srcLength = 0);
 
-  void uniformMatrix3x2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix3x2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                           optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
-  void uniformMatrix4x2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix4x2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                           optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
 
-  void uniformMatrix2x3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix2x3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                           optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
-  void uniformMatrix4x3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix4x3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                           optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
 
-  void uniformMatrix2x4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix2x4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                           optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
-  void uniformMatrix3x4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix3x4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                           optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
 
   /* Vertex attribs */
-  void vertexAttribI4i(GLuint index, GLint x, GLint y, GLint z, GLint w);
-  void vertexAttribI4iv(GLuint index, Int32List values);
-  void vertexAttribI4ui(GLuint index, GLuint x, GLuint y, GLuint z, GLuint w);
-  void vertexAttribI4uiv(GLuint index, Uint32List values);
-  void vertexAttribIPointer(GLuint index, GLint size, GLenum type, GLsizei stride, GLintptr offset);
+  undefined vertexAttribI4i(GLuint index, GLint x, GLint y, GLint z, GLint w);
+  undefined vertexAttribI4iv(GLuint index, Int32List values);
+  undefined vertexAttribI4ui(GLuint index, GLuint x, GLuint y, GLuint z, GLuint w);
+  undefined vertexAttribI4uiv(GLuint index, Uint32List values);
+  undefined vertexAttribIPointer(GLuint index, GLint size, GLenum type, GLsizei stride, GLintptr offset);
 
   /* Writing to the drawing buffer */
-  void vertexAttribDivisor(GLuint index, GLuint divisor);
-  void drawArraysInstanced(GLenum mode, GLint first, GLsizei count, GLsizei instanceCount);
-  void drawElementsInstanced(GLenum mode, GLsizei count, GLenum type, GLintptr offset, GLsizei instanceCount);
-  void drawRangeElements(GLenum mode, GLuint start, GLuint end, GLsizei count, GLenum type, GLintptr offset);
+  undefined vertexAttribDivisor(GLuint index, GLuint divisor);
+  undefined drawArraysInstanced(GLenum mode, GLint first, GLsizei count, GLsizei instanceCount);
+  undefined drawElementsInstanced(GLenum mode, GLsizei count, GLenum type, GLintptr offset, GLsizei instanceCount);
+  undefined drawRangeElements(GLenum mode, GLuint start, GLuint end, GLsizei count, GLenum type, GLintptr offset);
 
   /* Multiple Render Targets */
-  void drawBuffers(sequence<GLenum> buffers);
+  undefined drawBuffers(sequence<GLenum> buffers);
 
-  void clearBufferfv(GLenum buffer, GLint drawbuffer, Float32List values,
+  undefined clearBufferfv(GLenum buffer, GLint drawbuffer, Float32List values,
                      optional GLuint srcOffset = 0);
-  void clearBufferiv(GLenum buffer, GLint drawbuffer, Int32List values,
+  undefined clearBufferiv(GLenum buffer, GLint drawbuffer, Int32List values,
                      optional GLuint srcOffset = 0);
-  void clearBufferuiv(GLenum buffer, GLint drawbuffer, Uint32List values,
+  undefined clearBufferuiv(GLenum buffer, GLint drawbuffer, Uint32List values,
                       optional GLuint srcOffset = 0);
 
-  void clearBufferfi(GLenum buffer, GLint drawbuffer, GLfloat depth, GLint stencil);
+  undefined clearBufferfi(GLenum buffer, GLint drawbuffer, GLfloat depth, GLint stencil);
 
   /* Query Objects */
   WebGLQuery? createQuery();
-  void deleteQuery(WebGLQuery? query);
+  undefined deleteQuery(WebGLQuery? query);
   /*[WebGLHandlesContextLoss]*/ GLboolean isQuery(WebGLQuery? query);
-  void beginQuery(GLenum target, WebGLQuery query);
-  void endQuery(GLenum target);
+  undefined beginQuery(GLenum target, WebGLQuery query);
+  undefined endQuery(GLenum target);
   WebGLQuery? getQuery(GLenum target, GLenum pname);
   any getQueryParameter(WebGLQuery query, GLenum pname);
 
   /* Sampler Objects */
   WebGLSampler? createSampler();
-  void deleteSampler(WebGLSampler? sampler);
+  undefined deleteSampler(WebGLSampler? sampler);
   [WebGLHandlesContextLoss] GLboolean isSampler(WebGLSampler? sampler);
-  void bindSampler(GLuint unit, WebGLSampler? sampler);
-  void samplerParameteri(WebGLSampler sampler, GLenum pname, GLint param);
-  void samplerParameterf(WebGLSampler sampler, GLenum pname, GLfloat param);
+  undefined bindSampler(GLuint unit, WebGLSampler? sampler);
+  undefined samplerParameteri(WebGLSampler sampler, GLenum pname, GLint param);
+  undefined samplerParameterf(WebGLSampler sampler, GLenum pname, GLfloat param);
   any getSamplerParameter(WebGLSampler sampler, GLenum pname);
 
   /* Sync objects */
   WebGLSync? fenceSync(GLenum condition, GLbitfield flags);
   [WebGLHandlesContextLoss] GLboolean isSync(WebGLSync? sync);
-  void deleteSync(WebGLSync? sync);
+  undefined deleteSync(WebGLSync? sync);
   GLenum clientWaitSync(WebGLSync sync, GLbitfield flags, GLuint64 timeout);
-  void waitSync(WebGLSync sync, GLbitfield flags, GLint64 timeout);
+  undefined waitSync(WebGLSync sync, GLbitfield flags, GLint64 timeout);
   any getSyncParameter(WebGLSync sync, GLenum pname);
 
   /* Transform Feedback */
   WebGLTransformFeedback? createTransformFeedback();
-  void deleteTransformFeedback(WebGLTransformFeedback? tf);
+  undefined deleteTransformFeedback(WebGLTransformFeedback? tf);
   [WebGLHandlesContextLoss] GLboolean isTransformFeedback(WebGLTransformFeedback? tf);
-  void bindTransformFeedback (GLenum target, WebGLTransformFeedback? tf);
-  void beginTransformFeedback(GLenum primitiveMode);
-  void endTransformFeedback();
-  void transformFeedbackVaryings(WebGLProgram program, sequence<DOMString> varyings, GLenum bufferMode);
+  undefined bindTransformFeedback (GLenum target, WebGLTransformFeedback? tf);
+  undefined beginTransformFeedback(GLenum primitiveMode);
+  undefined endTransformFeedback();
+  undefined transformFeedbackVaryings(WebGLProgram program, sequence<DOMString> varyings, GLenum bufferMode);
   WebGLActiveInfo? getTransformFeedbackVarying(WebGLProgram program, GLuint index);
-  void pauseTransformFeedback();
-  void resumeTransformFeedback();
+  undefined pauseTransformFeedback();
+  undefined resumeTransformFeedback();
 
   /* Uniform Buffer Objects and Transform Feedback Buffers */
-  void bindBufferBase(GLenum target, GLuint index, WebGLBuffer? buffer);
-  void bindBufferRange(GLenum target, GLuint index, WebGLBuffer? buffer, GLintptr offset, GLsizeiptr size);
+  undefined bindBufferBase(GLenum target, GLuint index, WebGLBuffer? buffer);
+  undefined bindBufferRange(GLenum target, GLuint index, WebGLBuffer? buffer, GLintptr offset, GLsizeiptr size);
   any getIndexedParameter(GLenum target, GLuint index);
   sequence<GLuint>? getUniformIndices(WebGLProgram program, sequence<DOMString> uniformNames);
   any getActiveUniforms(WebGLProgram program, sequence<GLuint> uniformIndices, GLenum pname);
   GLuint getUniformBlockIndex(WebGLProgram program, DOMString uniformBlockName);
   any getActiveUniformBlockParameter(WebGLProgram program, GLuint uniformBlockIndex, GLenum pname);
   DOMString? getActiveUniformBlockName(WebGLProgram program, GLuint uniformBlockIndex);
-  void uniformBlockBinding(WebGLProgram program, GLuint uniformBlockIndex, GLuint uniformBlockBinding);
+  undefined uniformBlockBinding(WebGLProgram program, GLuint uniformBlockIndex, GLuint uniformBlockBinding);
 
   /* Vertex Array Objects */
   WebGLVertexArrayObject? createVertexArray();
-  void deleteVertexArray(WebGLVertexArrayObject? vertexArray);
+  undefined deleteVertexArray(WebGLVertexArrayObject? vertexArray);
   [WebGLHandlesContextLoss] GLboolean isVertexArray(WebGLVertexArrayObject? vertexArray);
-  void bindVertexArray(WebGLVertexArrayObject? array);
+  undefined bindVertexArray(WebGLVertexArrayObject? array);
 };
 
 interface mixin WebGL2RenderingContextOverloads
 {
   // WebGL1:
-  void bufferData(GLenum target, GLsizeiptr size, GLenum usage);
-  void bufferData(GLenum target, /*[AllowShared]*/ BufferSource? srcData, GLenum usage);
-  void bufferSubData(GLenum target, GLintptr dstByteOffset, /*[AllowShared]*/ BufferSource srcData);
+  undefined bufferData(GLenum target, GLsizeiptr size, GLenum usage);
+  undefined bufferData(GLenum target, /*[AllowShared]*/ BufferSource? srcData, GLenum usage);
+  undefined bufferSubData(GLenum target, GLintptr dstByteOffset, /*[AllowShared]*/ BufferSource srcData);
   // WebGL2:
-  void bufferData(GLenum target, /*[AllowShared]*/ ArrayBufferView srcData, GLenum usage, GLuint srcOffset,
+  undefined bufferData(GLenum target, /*[AllowShared]*/ ArrayBufferView srcData, GLenum usage, GLuint srcOffset,
                   optional GLuint length = 0);
-  void bufferSubData(GLenum target, GLintptr dstByteOffset, /*[AllowShared]*/ ArrayBufferView srcData,
+  undefined bufferSubData(GLenum target, GLintptr dstByteOffset, /*[AllowShared]*/ ArrayBufferView srcData,
                      GLuint srcOffset, optional GLuint length = 0);
 
   // WebGL1 legacy entrypoints:
   [Throws]
-  void texImage2D(GLenum target, GLint level, GLint internalformat,
+  undefined texImage2D(GLenum target, GLint level, GLint internalformat,
                   GLsizei width, GLsizei height, GLint border, GLenum format,
                   GLenum type, /*[AllowShared]*/ ArrayBufferView? pixels);
   [Throws]
-  void texImage2D(GLenum target, GLint level, GLint internalformat,
+  undefined texImage2D(GLenum target, GLint level, GLint internalformat,
                   GLenum format, GLenum type, TexImageSource source); // May throw DOMException
 
   [Throws]
-  void texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
+  undefined texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
                      GLsizei width, GLsizei height,
                      GLenum format, GLenum type, /*[AllowShared]*/ ArrayBufferView? pixels);
   [Throws]
-  void texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
+  undefined texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
                      GLenum format, GLenum type, TexImageSource source); // May throw DOMException
 
   // WebGL2 entrypoints:
   [Throws]
-  void texImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height,
+  undefined texImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height,
                   GLint border, GLenum format, GLenum type, GLintptr pboOffset);
   [Throws]
-  void texImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height,
+  undefined texImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height,
                   GLint border, GLenum format, GLenum type,
                   TexImageSource source); // May throw DOMException
   [Throws]
-  void texImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height,
+  undefined texImage2D(GLenum target, GLint level, GLint internalformat, GLsizei width, GLsizei height,
                   GLint border, GLenum format, GLenum type, /*[AllowShared]*/ ArrayBufferView srcData,
                   GLuint srcOffset);
 
@@ -531,51 +531,51 @@ interface mixin WebGL2RenderingContextOverloads
 
   //void compressedTexImage2D(GLenum target, GLint level, GLenum internalformat, GLsizei width,
   //                          GLsizei height, GLint border, GLsizei imageSize, GLintptr offset);
-  void compressedTexImage2D(GLenum target, GLint level, GLenum internalformat, GLsizei width,
+  undefined compressedTexImage2D(GLenum target, GLint level, GLenum internalformat, GLsizei width,
                             GLsizei height, GLint border, /*[AllowShared]*/ ArrayBufferView srcData,
                             optional GLuint srcOffset = 0, optional GLuint srcLengthOverride = 0);
 
   //void compressedTexSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
   //                             GLsizei width, GLsizei height, GLenum format, GLsizei imageSize, GLintptr offset);
-  void compressedTexSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
+  undefined compressedTexSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
                                GLsizei width, GLsizei height, GLenum format,
                                /*[AllowShared]*/ ArrayBufferView srcData,
                                optional GLuint srcOffset = 0,
                                optional GLuint srcLengthOverride = 0);
 
-  void uniform1fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
+  undefined uniform1fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
-  void uniform2fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
+  undefined uniform2fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
-  void uniform3fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
+  undefined uniform3fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
-  void uniform4fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
+  undefined uniform4fv(WebGLUniformLocation? location, Float32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
 
-  void uniform1iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
+  undefined uniform1iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
-  void uniform2iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
+  undefined uniform2iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
-  void uniform3iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
+  undefined uniform3iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
-  void uniform4iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
+  undefined uniform4iv(WebGLUniformLocation? location, Int32List data, optional GLuint srcOffset = 0,
                   optional GLuint srcLength = 0);
 
-  void uniformMatrix2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                         optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
-  void uniformMatrix3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                         optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
-  void uniformMatrix4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
+  undefined uniformMatrix4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List data,
                         optional GLuint srcOffset = 0, optional GLuint srcLength = 0);
 
   /* Reading back pixels */
   // WebGL1:
-  void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type,
+  undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type,
                   /*[AllowShared]*/ ArrayBufferView? dstData);
   // WebGL2:
-  void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type,
+  undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type,
                   GLintptr offset);
-  void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type,
+  undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type,
                   /*[AllowShared]*/ ArrayBufferView dstData, GLuint dstOffset);
 };
 
diff --git a/components/script/dom/webidls/WebGLRenderingContext.webidl b/components/script/dom/webidls/WebGLRenderingContext.webidl
index 4003ab86ba5..f54dfd67507 100644
--- a/components/script/dom/webidls/WebGLRenderingContext.webidl
+++ b/components/script/dom/webidls/WebGLRenderingContext.webidl
@@ -475,32 +475,32 @@ interface mixin WebGLRenderingContextBase
     sequence<DOMString>? getSupportedExtensions();
     object? getExtension(DOMString name);
 
-    void activeTexture(GLenum texture);
-    void attachShader(WebGLProgram program, WebGLShader shader);
-    void bindAttribLocation(WebGLProgram program, GLuint index, DOMString name);
-    void bindBuffer(GLenum target, WebGLBuffer? buffer);
-    void bindFramebuffer(GLenum target, WebGLFramebuffer? framebuffer);
-    void bindRenderbuffer(GLenum target, WebGLRenderbuffer? renderbuffer);
-    void bindTexture(GLenum target, WebGLTexture? texture);
-    void blendColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);
-    void blendEquation(GLenum mode);
-    void blendEquationSeparate(GLenum modeRGB, GLenum modeAlpha);
-    void blendFunc(GLenum sfactor, GLenum dfactor);
-    void blendFuncSeparate(GLenum srcRGB, GLenum dstRGB,
+    undefined activeTexture(GLenum texture);
+    undefined attachShader(WebGLProgram program, WebGLShader shader);
+    undefined bindAttribLocation(WebGLProgram program, GLuint index, DOMString name);
+    undefined bindBuffer(GLenum target, WebGLBuffer? buffer);
+    undefined bindFramebuffer(GLenum target, WebGLFramebuffer? framebuffer);
+    undefined bindRenderbuffer(GLenum target, WebGLRenderbuffer? renderbuffer);
+    undefined bindTexture(GLenum target, WebGLTexture? texture);
+    undefined blendColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);
+    undefined blendEquation(GLenum mode);
+    undefined blendEquationSeparate(GLenum modeRGB, GLenum modeAlpha);
+    undefined blendFunc(GLenum sfactor, GLenum dfactor);
+    undefined blendFuncSeparate(GLenum srcRGB, GLenum dstRGB,
                            GLenum srcAlpha, GLenum dstAlpha);
 
     [WebGLHandlesContextLoss] GLenum checkFramebufferStatus(GLenum target);
-    void clear(GLbitfield mask);
-    void clearColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);
-    void clearDepth(GLclampf depth);
-    void clearStencil(GLint s);
-    void colorMask(GLboolean red, GLboolean green, GLboolean blue, GLboolean alpha);
-    void compileShader(WebGLShader shader);
-
-    void copyTexImage2D(GLenum target, GLint level, GLenum internalformat,
+    undefined clear(GLbitfield mask);
+    undefined clearColor(GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);
+    undefined clearDepth(GLclampf depth);
+    undefined clearStencil(GLint s);
+    undefined colorMask(GLboolean red, GLboolean green, GLboolean blue, GLboolean alpha);
+    undefined compileShader(WebGLShader shader);
+
+    undefined copyTexImage2D(GLenum target, GLint level, GLenum internalformat,
                         GLint x, GLint y, GLsizei width, GLsizei height,
                         GLint border);
-    void copyTexSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
+    undefined copyTexSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
                            GLint x, GLint y, GLsizei width, GLsizei height);
 
     WebGLBuffer? createBuffer();
@@ -510,36 +510,36 @@ interface mixin WebGLRenderingContextBase
     WebGLShader? createShader(GLenum type);
     WebGLTexture? createTexture();
 
-    void cullFace(GLenum mode);
-
-    void deleteBuffer(WebGLBuffer? buffer);
-    void deleteFramebuffer(WebGLFramebuffer? framebuffer);
-    void deleteProgram(WebGLProgram? program);
-    void deleteRenderbuffer(WebGLRenderbuffer? renderbuffer);
-    void deleteShader(WebGLShader? shader);
-    void deleteTexture(WebGLTexture? texture);
-
-    void depthFunc(GLenum func);
-    void depthMask(GLboolean flag);
-    void depthRange(GLclampf zNear, GLclampf zFar);
-    void detachShader(WebGLProgram program, WebGLShader shader);
-    void disable(GLenum cap);
-    void disableVertexAttribArray(GLuint index);
-    void drawArrays(GLenum mode, GLint first, GLsizei count);
-    void drawElements(GLenum mode, GLsizei count, GLenum type, GLintptr offset);
-
-    void enable(GLenum cap);
-    void enableVertexAttribArray(GLuint index);
-    void finish();
-    void flush();
-    void framebufferRenderbuffer(GLenum target, GLenum attachment,
+    undefined cullFace(GLenum mode);
+
+    undefined deleteBuffer(WebGLBuffer? buffer);
+    undefined deleteFramebuffer(WebGLFramebuffer? framebuffer);
+    undefined deleteProgram(WebGLProgram? program);
+    undefined deleteRenderbuffer(WebGLRenderbuffer? renderbuffer);
+    undefined deleteShader(WebGLShader? shader);
+    undefined deleteTexture(WebGLTexture? texture);
+
+    undefined depthFunc(GLenum func);
+    undefined depthMask(GLboolean flag);
+    undefined depthRange(GLclampf zNear, GLclampf zFar);
+    undefined detachShader(WebGLProgram program, WebGLShader shader);
+    undefined disable(GLenum cap);
+    undefined disableVertexAttribArray(GLuint index);
+    undefined drawArrays(GLenum mode, GLint first, GLsizei count);
+    undefined drawElements(GLenum mode, GLsizei count, GLenum type, GLintptr offset);
+
+    undefined enable(GLenum cap);
+    undefined enableVertexAttribArray(GLuint index);
+    undefined finish();
+    undefined flush();
+    undefined framebufferRenderbuffer(GLenum target, GLenum attachment,
                                  GLenum renderbuffertarget,
                                  WebGLRenderbuffer? renderbuffer);
-    void framebufferTexture2D(GLenum target, GLenum attachment, GLenum textarget,
+    undefined framebufferTexture2D(GLenum target, GLenum attachment, GLenum textarget,
                               WebGLTexture? texture, GLint level);
-    void frontFace(GLenum mode);
+    undefined frontFace(GLenum mode);
 
-    void generateMipmap(GLenum target);
+    undefined generateMipmap(GLenum target);
 
     WebGLActiveInfo? getActiveAttrib(WebGLProgram program, GLuint index);
     WebGLActiveInfo? getActiveUniform(WebGLProgram program, GLuint index);
@@ -573,7 +573,7 @@ interface mixin WebGLRenderingContextBase
 
     [WebGLHandlesContextLoss] GLsizeiptr getVertexAttribOffset(GLuint index, GLenum pname);
 
-    void hint(GLenum target, GLenum mode);
+    undefined hint(GLenum target, GLenum mode);
     [WebGLHandlesContextLoss] GLboolean isBuffer(WebGLBuffer? buffer);
     [WebGLHandlesContextLoss] GLboolean isEnabled(GLenum cap);
     [WebGLHandlesContextLoss] GLboolean isFramebuffer(WebGLFramebuffer? framebuffer);
@@ -581,108 +581,108 @@ interface mixin WebGLRenderingContextBase
     [WebGLHandlesContextLoss] GLboolean isRenderbuffer(WebGLRenderbuffer? renderbuffer);
     [WebGLHandlesContextLoss] GLboolean isShader(WebGLShader? shader);
     [WebGLHandlesContextLoss] GLboolean isTexture(WebGLTexture? texture);
-    void lineWidth(GLfloat width);
-    void linkProgram(WebGLProgram program);
-    void pixelStorei(GLenum pname, GLint param);
-    void polygonOffset(GLfloat factor, GLfloat units);
+    undefined lineWidth(GLfloat width);
+    undefined linkProgram(WebGLProgram program);
+    undefined pixelStorei(GLenum pname, GLint param);
+    undefined polygonOffset(GLfloat factor, GLfloat units);
 
-    void renderbufferStorage(GLenum target, GLenum internalformat,
+    undefined renderbufferStorage(GLenum target, GLenum internalformat,
                              GLsizei width, GLsizei height);
-    void sampleCoverage(GLclampf value, GLboolean invert);
-    void scissor(GLint x, GLint y, GLsizei width, GLsizei height);
+    undefined sampleCoverage(GLclampf value, GLboolean invert);
+    undefined scissor(GLint x, GLint y, GLsizei width, GLsizei height);
 
-    void shaderSource(WebGLShader shader, DOMString source);
+    undefined shaderSource(WebGLShader shader, DOMString source);
 
-    void stencilFunc(GLenum func, GLint ref, GLuint mask);
-    void stencilFuncSeparate(GLenum face, GLenum func, GLint ref, GLuint mask);
-    void stencilMask(GLuint mask);
-    void stencilMaskSeparate(GLenum face, GLuint mask);
-    void stencilOp(GLenum fail, GLenum zfail, GLenum zpass);
-    void stencilOpSeparate(GLenum face, GLenum fail, GLenum zfail, GLenum zpass);
+    undefined stencilFunc(GLenum func, GLint ref, GLuint mask);
+    undefined stencilFuncSeparate(GLenum face, GLenum func, GLint ref, GLuint mask);
+    undefined stencilMask(GLuint mask);
+    undefined stencilMaskSeparate(GLenum face, GLuint mask);
+    undefined stencilOp(GLenum fail, GLenum zfail, GLenum zpass);
+    undefined stencilOpSeparate(GLenum face, GLenum fail, GLenum zfail, GLenum zpass);
 
-    void texParameterf(GLenum target, GLenum pname, GLfloat param);
-    void texParameteri(GLenum target, GLenum pname, GLint param);
+    undefined texParameterf(GLenum target, GLenum pname, GLfloat param);
+    undefined texParameteri(GLenum target, GLenum pname, GLint param);
 
-    void uniform1f(WebGLUniformLocation? location, GLfloat x);
-    void uniform2f(WebGLUniformLocation? location, GLfloat x, GLfloat y);
-    void uniform3f(WebGLUniformLocation? location, GLfloat x, GLfloat y, GLfloat z);
-    void uniform4f(WebGLUniformLocation? location, GLfloat x, GLfloat y, GLfloat z, GLfloat w);
+    undefined uniform1f(WebGLUniformLocation? location, GLfloat x);
+    undefined uniform2f(WebGLUniformLocation? location, GLfloat x, GLfloat y);
+    undefined uniform3f(WebGLUniformLocation? location, GLfloat x, GLfloat y, GLfloat z);
+    undefined uniform4f(WebGLUniformLocation? location, GLfloat x, GLfloat y, GLfloat z, GLfloat w);
 
-    void uniform1i(WebGLUniformLocation? location, GLint x);
-    void uniform2i(WebGLUniformLocation? location, GLint x, GLint y);
-    void uniform3i(WebGLUniformLocation? location, GLint x, GLint y, GLint z);
-    void uniform4i(WebGLUniformLocation? location, GLint x, GLint y, GLint z, GLint w);
+    undefined uniform1i(WebGLUniformLocation? location, GLint x);
+    undefined uniform2i(WebGLUniformLocation? location, GLint x, GLint y);
+    undefined uniform3i(WebGLUniformLocation? location, GLint x, GLint y, GLint z);
+    undefined uniform4i(WebGLUniformLocation? location, GLint x, GLint y, GLint z, GLint w);
 
-    void useProgram(WebGLProgram? program);
-    void validateProgram(WebGLProgram program);
+    undefined useProgram(WebGLProgram? program);
+    undefined validateProgram(WebGLProgram program);
 
-    void vertexAttrib1f(GLuint indx, GLfloat x);
-    void vertexAttrib2f(GLuint indx, GLfloat x, GLfloat y);
-    void vertexAttrib3f(GLuint indx, GLfloat x, GLfloat y, GLfloat z);
-    void vertexAttrib4f(GLuint indx, GLfloat x, GLfloat y, GLfloat z, GLfloat w);
+    undefined vertexAttrib1f(GLuint indx, GLfloat x);
+    undefined vertexAttrib2f(GLuint indx, GLfloat x, GLfloat y);
+    undefined vertexAttrib3f(GLuint indx, GLfloat x, GLfloat y, GLfloat z);
+    undefined vertexAttrib4f(GLuint indx, GLfloat x, GLfloat y, GLfloat z, GLfloat w);
 
-    void vertexAttrib1fv(GLuint indx, Float32List values);
-    void vertexAttrib2fv(GLuint indx, Float32List values);
-    void vertexAttrib3fv(GLuint indx, Float32List values);
-    void vertexAttrib4fv(GLuint indx, Float32List values);
+    undefined vertexAttrib1fv(GLuint indx, Float32List values);
+    undefined vertexAttrib2fv(GLuint indx, Float32List values);
+    undefined vertexAttrib3fv(GLuint indx, Float32List values);
+    undefined vertexAttrib4fv(GLuint indx, Float32List values);
 
-    void vertexAttribPointer(GLuint indx, GLint size, GLenum type,
+    undefined vertexAttribPointer(GLuint indx, GLint size, GLenum type,
                              GLboolean normalized, GLsizei stride, GLintptr offset);
 
-    void viewport(GLint x, GLint y, GLsizei width, GLsizei height);
+    undefined viewport(GLint x, GLint y, GLsizei width, GLsizei height);
 };
 
 interface mixin WebGLRenderingContextOverloads
 {
-    void bufferData(GLenum target, GLsizeiptr size, GLenum usage);
-    void bufferData(GLenum target, /*[AllowShared]*/ BufferSource? data, GLenum usage);
-    void bufferSubData(GLenum target, GLintptr offset, /*[AllowShared]*/ BufferSource data);
+    undefined bufferData(GLenum target, GLsizeiptr size, GLenum usage);
+    undefined bufferData(GLenum target, /*[AllowShared]*/ BufferSource? data, GLenum usage);
+    undefined bufferSubData(GLenum target, GLintptr offset, /*[AllowShared]*/ BufferSource data);
 
-    void compressedTexImage2D(GLenum target, GLint level, GLenum internalformat,
+    undefined compressedTexImage2D(GLenum target, GLint level, GLenum internalformat,
                               GLsizei width, GLsizei height, GLint border,
                               /*[AllowShared]*/ ArrayBufferView data);
-    void compressedTexSubImage2D(GLenum target, GLint level,
+    undefined compressedTexSubImage2D(GLenum target, GLint level,
                                  GLint xoffset, GLint yoffset,
                                  GLsizei width, GLsizei height, GLenum format,
                                  /*[AllowShared]*/ ArrayBufferView data);
 
-    void readPixels(GLint x, GLint y, GLsizei width, GLsizei height,
+    undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height,
                     GLenum format, GLenum type, /*[AllowShared]*/ ArrayBufferView? pixels);
 
     [Throws]
-    void texImage2D(GLenum target, GLint level, GLint internalformat,
+    undefined texImage2D(GLenum target, GLint level, GLint internalformat,
                     GLsizei width, GLsizei height, GLint border, GLenum format,
                     GLenum type, /*[AllowShared]*/ ArrayBufferView? pixels);
     [Throws]
-    void texImage2D(GLenum target, GLint level, GLint internalformat,
+    undefined texImage2D(GLenum target, GLint level, GLint internalformat,
                     GLenum format, GLenum type, TexImageSource source); // May throw DOMException
 
     [Throws]
-    void texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
+    undefined texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
                        GLsizei width, GLsizei height,
                        GLenum format, GLenum type, /*[AllowShared]*/ ArrayBufferView? pixels);
     [Throws]
-    void texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
+    undefined texSubImage2D(GLenum target, GLint level, GLint xoffset, GLint yoffset,
                        GLenum format, GLenum type, TexImageSource source); // May throw DOMException
 
-    void uniform1fv(WebGLUniformLocation? location, Float32List v);
-    void uniform2fv(WebGLUniformLocation? location, Float32List v);
-    void uniform3fv(WebGLUniformLocation? location, Float32List v);
-    void uniform4fv(WebGLUniformLocation? location, Float32List v);
+    undefined uniform1fv(WebGLUniformLocation? location, Float32List v);
+    undefined uniform2fv(WebGLUniformLocation? location, Float32List v);
+    undefined uniform3fv(WebGLUniformLocation? location, Float32List v);
+    undefined uniform4fv(WebGLUniformLocation? location, Float32List v);
 
-    void uniform1iv(WebGLUniformLocation? location, Int32List v);
-    void uniform2iv(WebGLUniformLocation? location, Int32List v);
-    void uniform3iv(WebGLUniformLocation? location, Int32List v);
-    void uniform4iv(WebGLUniformLocation? location, Int32List v);
+    undefined uniform1iv(WebGLUniformLocation? location, Int32List v);
+    undefined uniform2iv(WebGLUniformLocation? location, Int32List v);
+    undefined uniform3iv(WebGLUniformLocation? location, Int32List v);
+    undefined uniform4iv(WebGLUniformLocation? location, Int32List v);
 
-    void uniformMatrix2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List value);
-    void uniformMatrix3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List value);
-    void uniformMatrix4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List value);
+    undefined uniformMatrix2fv(WebGLUniformLocation? location, GLboolean transpose, Float32List value);
+    undefined uniformMatrix3fv(WebGLUniformLocation? location, GLboolean transpose, Float32List value);
+    undefined uniformMatrix4fv(WebGLUniformLocation? location, GLboolean transpose, Float32List value);
 };
 
 interface mixin WebGLRenderingContextExtensions {
     [Throws, Pref="dom.webgl.dom_to_texture.enabled"]
-    void texImageDOM(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height,
+    undefined texImageDOM(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height,
                      GLenum format, GLenum type, HTMLIFrameElement source); // May throw DOMException
 };
 
diff --git a/components/script/dom/webidls/WebSocket.webidl b/components/script/dom/webidls/WebSocket.webidl
index 089b2c5a378..9fff327fc6f 100644
--- a/components/script/dom/webidls/WebSocket.webidl
+++ b/components/script/dom/webidls/WebSocket.webidl
@@ -24,13 +24,13 @@ interface WebSocket : EventTarget {
     attribute EventHandler onclose;
     //readonly attribute DOMString extensions;
     readonly attribute DOMString protocol;
-    [Throws] void close(optional [Clamp] unsigned short code, optional USVString reason);
+    [Throws] undefined close(optional [Clamp] unsigned short code, optional USVString reason);
 
     //messaging
     attribute EventHandler onmessage;
     attribute BinaryType binaryType;
-    [Throws] void send(USVString data);
-    [Throws] void send(Blob data);
-    [Throws] void send(ArrayBuffer data);
-    [Throws] void send(ArrayBufferView data);
+    [Throws] undefined send(USVString data);
+    [Throws] undefined send(Blob data);
+    [Throws] undefined send(ArrayBuffer data);
+    [Throws] undefined send(ArrayBufferView data);
 };
diff --git a/components/script/dom/webidls/WheelEvent.webidl b/components/script/dom/webidls/WheelEvent.webidl
index 15fc033a21b..7667aa81560 100644
--- a/components/script/dom/webidls/WheelEvent.webidl
+++ b/components/script/dom/webidls/WheelEvent.webidl
@@ -26,7 +26,7 @@ dictionary WheelEventInit : MouseEventInit {
 // https://w3c.github.io/uievents/#idl-interface-WheelEvent-initializers
 partial interface WheelEvent {
     // Deprecated in DOM Level 3
-    void initWheelEvent (DOMString typeArg, boolean bubblesArg, boolean cancelableArg,
+    undefined initWheelEvent (DOMString typeArg, boolean bubblesArg, boolean cancelableArg,
                          Window? viewArg, long detailArg,
                          double deltaX, double deltaY,
                          double deltaZ, unsigned long deltaMode);
diff --git a/components/script/dom/webidls/Window.webidl b/components/script/dom/webidls/Window.webidl
index 289aa52cfb8..23aaa8fa07d 100644
--- a/components/script/dom/webidls/Window.webidl
+++ b/components/script/dom/webidls/Window.webidl
@@ -24,9 +24,9 @@
   //[Replaceable] readonly attribute BarProp statusbar;
   //[Replaceable] readonly attribute BarProp toolbar;
   attribute DOMString status;
-  [CrossOriginCallable] void close();
+  [CrossOriginCallable] undefined close();
   [CrossOriginReadable] readonly attribute boolean closed;
-  void stop();
+  undefined stop();
   //[CrossOriginCallable] void focus();
   //[CrossOriginCallable] void blur();
 
@@ -54,20 +54,20 @@
   //readonly attribute ApplicationCache applicationCache;
 
   // user prompts
-  void alert(DOMString message);
-  void alert();
+  undefined alert(DOMString message);
+  undefined alert();
   boolean confirm(optional DOMString message = "");
   DOMString? prompt(optional DOMString message = "", optional DOMString default = "");
   //void print();
   //any showModalDialog(DOMString url, optional any argument);
 
   unsigned long requestAnimationFrame(FrameRequestCallback callback);
-  void cancelAnimationFrame(unsigned long handle);
+  undefined cancelAnimationFrame(unsigned long handle);
 
   [Throws, CrossOriginCallable]
-  void postMessage(any message, USVString targetOrigin, optional sequence<object> transfer = []);
+  undefined postMessage(any message, USVString targetOrigin, optional sequence<object> transfer = []);
   [Throws, CrossOriginCallable]
-  void postMessage(any message, optional WindowPostMessageOptions options = {});
+  undefined postMessage(any message, optional WindowPostMessageOptions options = {});
 
   // also has obsolete members
 };
@@ -76,8 +76,8 @@ Window includes WindowEventHandlers;
 
 // https://html.spec.whatwg.org/multipage/#Window-partial
 partial interface Window {
-  void captureEvents();
-  void releaseEvents();
+  undefined captureEvents();
+  undefined releaseEvents();
 };
 
 // https://drafts.csswg.org/cssom/#extensions-to-the-window-interface
@@ -106,10 +106,10 @@ partial interface Window {
   [SameObject, Replaceable] readonly attribute Screen screen;
 
   // browsing context
-  void moveTo(long x, long y);
-  void moveBy(long x, long y);
-  void resizeTo(long x, long y);
-  void resizeBy(long x, long y);
+  undefined moveTo(long x, long y);
+  undefined moveBy(long x, long y);
+  undefined resizeTo(long x, long y);
+  undefined resizeBy(long x, long y);
 
   // viewport
   [Replaceable] readonly attribute long innerWidth;
@@ -120,12 +120,12 @@ partial interface Window {
   [Replaceable] readonly attribute long pageXOffset;
   [Replaceable] readonly attribute long scrollY;
   [Replaceable] readonly attribute long pageYOffset;
-  void scroll(optional ScrollToOptions options = {});
-  void scroll(unrestricted double x, unrestricted double y);
-  void scrollTo(optional ScrollToOptions options = {});
-  void scrollTo(unrestricted double x, unrestricted double y);
-  void scrollBy(optional ScrollToOptions options = {});
-  void scrollBy(unrestricted double x, unrestricted double y);
+  undefined scroll(optional ScrollToOptions options = {});
+  undefined scroll(unrestricted double x, unrestricted double y);
+  undefined scrollTo(optional ScrollToOptions options = {});
+  undefined scrollTo(unrestricted double x, unrestricted double y);
+  undefined scrollBy(optional ScrollToOptions options = {});
+  undefined scrollBy(unrestricted double x, unrestricted double y);
 
   // client
   [Replaceable] readonly attribute long screenX;
@@ -138,20 +138,20 @@ partial interface Window {
 // Proprietary extensions.
 partial interface Window {
   [Pref="dom.servo_helpers.enabled"]
-  void debug(DOMString arg);
+  undefined debug(DOMString arg);
   [Pref="dom.servo_helpers.enabled"]
-  void gc();
+  undefined gc();
   [Pref="dom.servo_helpers.enabled"]
-  void trap();
+  undefined trap();
   [Pref="dom.servo_helpers.enabled"]
-  void js_backtrace();
+  undefined js_backtrace();
 };
 
 // WebDriver extensions
 partial interface Window {
   // Shouldn't be public, but just to make things work for now
-  void webdriverCallback(optional any result);
-  void webdriverTimeout();
+  undefined webdriverCallback(optional any result);
+  undefined webdriverTimeout();
 };
 
 // https://html.spec.whatwg.org/multipage/#dom-sessionstorage
@@ -167,7 +167,7 @@ interface mixin WindowLocalStorage {
 Window includes WindowLocalStorage;
 
 // http://w3c.github.io/animation-timing/#framerequestcallback
-callback FrameRequestCallback = void (DOMHighResTimeStamp time);
+callback FrameRequestCallback = undefined (DOMHighResTimeStamp time);
 
 // https://webbluetoothcg.github.io/web-bluetooth/tests#test-interfaces
 partial interface Window {
diff --git a/components/script/dom/webidls/WindowOrWorkerGlobalScope.webidl b/components/script/dom/webidls/WindowOrWorkerGlobalScope.webidl
index 2ab8ba2c25d..0d28857ec58 100644
--- a/components/script/dom/webidls/WindowOrWorkerGlobalScope.webidl
+++ b/components/script/dom/webidls/WindowOrWorkerGlobalScope.webidl
@@ -16,12 +16,12 @@ interface mixin WindowOrWorkerGlobalScope {
 
   // timers
   long setTimeout(TimerHandler handler, optional long timeout = 0, any... arguments);
-  void clearTimeout(optional long handle = 0);
+  undefined clearTimeout(optional long handle = 0);
   long setInterval(TimerHandler handler, optional long timeout = 0, any... arguments);
-  void clearInterval(optional long handle = 0);
+  undefined clearInterval(optional long handle = 0);
 
   // microtask queuing
-  void queueMicrotask(VoidFunction callback);
+  undefined queueMicrotask(VoidFunction callback);
 
   // ImageBitmap
   [Pref="dom.imagebitmap.enabled"]
diff --git a/components/script/dom/webidls/Worker.webidl b/components/script/dom/webidls/Worker.webidl
index 93df577ec32..b391d65b2ab 100644
--- a/components/script/dom/webidls/Worker.webidl
+++ b/components/script/dom/webidls/Worker.webidl
@@ -12,10 +12,10 @@ interface mixin AbstractWorker {
 [Exposed=(Window,Worker)]
 interface Worker : EventTarget {
   [Throws] constructor(USVString scriptURL, optional WorkerOptions options = {});
-  void terminate();
+  undefined terminate();
 
-  [Throws] void postMessage(any message, sequence<object> transfer);
-  [Throws] void postMessage(any message, optional PostMessageOptions options = {});
+  [Throws] undefined postMessage(any message, sequence<object> transfer);
+  [Throws] undefined postMessage(any message, optional PostMessageOptions options = {});
   attribute EventHandler onmessage;
   attribute EventHandler onmessageerror;
 };
diff --git a/components/script/dom/webidls/WorkerGlobalScope.webidl b/components/script/dom/webidls/WorkerGlobalScope.webidl
index 191ab63967d..05301fb28e6 100644
--- a/components/script/dom/webidls/WorkerGlobalScope.webidl
+++ b/components/script/dom/webidls/WorkerGlobalScope.webidl
@@ -19,6 +19,6 @@ interface WorkerGlobalScope : GlobalScope {
 [Exposed=Worker]
 partial interface WorkerGlobalScope { // not obsolete
   [Throws]
-  void importScripts(DOMString... urls);
+  undefined importScripts(DOMString... urls);
   readonly attribute WorkerNavigator navigator;
 };
diff --git a/components/script/dom/webidls/Worklet.webidl b/components/script/dom/webidls/Worklet.webidl
index 16ec8e8d7c9..249f9c9d470 100644
--- a/components/script/dom/webidls/Worklet.webidl
+++ b/components/script/dom/webidls/Worklet.webidl
@@ -5,7 +5,7 @@
 // https://drafts.css-houdini.org/worklets/#worklet
 [Pref="dom.worklet.enabled", Exposed=(Window)]
 interface Worklet {
-    [NewObject] Promise<void> addModule(USVString moduleURL, optional WorkletOptions options = {});
+    [NewObject] Promise<undefined> addModule(USVString moduleURL, optional WorkletOptions options = {});
 };
 
 dictionary WorkletOptions {
diff --git a/components/script/dom/webidls/XMLHttpRequest.webidl b/components/script/dom/webidls/XMLHttpRequest.webidl
index 2c043b9407a..c7225cff112 100644
--- a/components/script/dom/webidls/XMLHttpRequest.webidl
+++ b/components/script/dom/webidls/XMLHttpRequest.webidl
@@ -43,22 +43,22 @@ interface XMLHttpRequest : XMLHttpRequestEventTarget {
 
   // request
   [Throws]
-  void open(ByteString method, USVString url);
+  undefined open(ByteString method, USVString url);
   [Throws]
-  void open(ByteString method, USVString url, boolean async,
+  undefined open(ByteString method, USVString url, boolean async,
             optional USVString? username = null,
             optional USVString? password = null);
 
   [Throws]
-  void setRequestHeader(ByteString name, ByteString value);
+  undefined setRequestHeader(ByteString name, ByteString value);
   [SetterThrows]
            attribute unsigned long timeout;
   [SetterThrows]
            attribute boolean withCredentials;
   readonly attribute XMLHttpRequestUpload upload;
   [Throws]
-  void send(optional (Document or XMLHttpRequestBodyInit)? data = null);
-  void abort();
+  undefined send(optional (Document or XMLHttpRequestBodyInit)? data = null);
+  undefined abort();
 
   // response
   readonly attribute USVString responseURL;
@@ -67,7 +67,7 @@ interface XMLHttpRequest : XMLHttpRequestEventTarget {
   ByteString? getResponseHeader(ByteString name);
   ByteString getAllResponseHeaders();
   [Throws]
-  void overrideMimeType(DOMString mime);
+  undefined overrideMimeType(DOMString mime);
   [SetterThrows]
            attribute XMLHttpRequestResponseType responseType;
   readonly attribute any response;
diff --git a/components/script/dom/webidls/XRHitTestSource.webidl b/components/script/dom/webidls/XRHitTestSource.webidl
index a3a56cebed4..e2a11768158 100644
--- a/components/script/dom/webidls/XRHitTestSource.webidl
+++ b/components/script/dom/webidls/XRHitTestSource.webidl
@@ -18,5 +18,5 @@ dictionary XRHitTestOptionsInit {
 
 [SecureContext, Exposed=Window]
 interface XRHitTestSource {
-  void cancel();
+  undefined cancel();
 };
diff --git a/components/script/dom/webidls/XRSession.webidl b/components/script/dom/webidls/XRSession.webidl
index 45cdbe2339e..89e6dc5c68c 100644
--- a/components/script/dom/webidls/XRSession.webidl
+++ b/components/script/dom/webidls/XRSession.webidl
@@ -16,7 +16,7 @@ enum XRVisibilityState {
   "hidden",
 };
 
-callback XRFrameRequestCallback = void (DOMHighResTimeStamp time, XRFrame frame);
+callback XRFrameRequestCallback = undefined (DOMHighResTimeStamp time, XRFrame frame);
 
 [SecureContext, Exposed=Window, Pref="dom.webxr.enabled"]
 interface XRSession : EventTarget {
@@ -28,13 +28,13 @@ interface XRSession : EventTarget {
   [SameObject] readonly attribute XRInputSourceArray inputSources;
 
   // // Methods
-  [Throws] void updateRenderState(optional XRRenderStateInit state = {});
+  [Throws] undefined updateRenderState(optional XRRenderStateInit state = {});
   Promise<XRReferenceSpace> requestReferenceSpace(XRReferenceSpaceType type);
 
   long requestAnimationFrame(XRFrameRequestCallback callback);
-  void cancelAnimationFrame(long handle);
+  undefined cancelAnimationFrame(long handle);
 
-  Promise<void> end();
+  Promise<undefined> end();
 
   // hit test module
   Promise<XRHitTestSource> requestHitTestSource(XRHitTestOptionsInit options);
diff --git a/components/script/dom/webidls/XRTest.webidl b/components/script/dom/webidls/XRTest.webidl
index 1ae98f703bf..b65fb1c7485 100644
--- a/components/script/dom/webidls/XRTest.webidl
+++ b/components/script/dom/webidls/XRTest.webidl
@@ -13,10 +13,10 @@ interface XRTest {
   // // Simulates a user activation (aka user gesture) for the current scope.
   // // The activation is only guaranteed to be valid in the provided function and only applies to WebXR
   // // Device API methods.
-  void simulateUserActivation(Function f);
+  undefined simulateUserActivation(Function f);
 
   // // Disconnect all fake devices
-  Promise<void> disconnectAllDevices();
+  Promise<undefined> disconnectAllDevices();
 };
 
 dictionary FakeXRDeviceInit {
diff --git a/components/script/dom/webidls/XRWebGLLayer.webidl b/components/script/dom/webidls/XRWebGLLayer.webidl
index 87d2dc4dccb..4a89ec2a40a 100644
--- a/components/script/dom/webidls/XRWebGLLayer.webidl
+++ b/components/script/dom/webidls/XRWebGLLayer.webidl
@@ -37,5 +37,5 @@ interface XRWebGLLayer: XRLayer {
 };
 
 partial interface WebGLRenderingContext {
-    [Pref="dom.webxr.enabled"] Promise<void> makeXRCompatible();
+    [Pref="dom.webxr.enabled"] Promise<undefined> makeXRCompatible();
 };