--- WebIDL.py
+++ WebIDL.py
@@ -1362,12 +1362,6 @@ class IDLInterfaceOrNamespace(IDLObjectWithScope, IDLExposureMixins):
                 for bindingAlias in member.bindingAliases:
                     checkDuplicateNames(member, bindingAlias, "BindingAlias")
 
-
-        if self.getExtendedAttribute("Pref") and self.isExposedOffMainThread():
-            raise WebIDLError("[Pref] used on an interface that is not "
-                              "main-thread-only",
-                              [self.location])
-
         # Conditional exposure makes no sense for interfaces with no
         # interface object, unless they're navigator properties.
         # And SecureContext makes sense for interfaces with no interface object,
@@ -3619,11 +3613,6 @@ class IDLInterfaceMember(IDLObjectWithIdentifier, IDLExposureMixins):
         IDLExposureMixins.finish(self, scope)
 
     def validate(self):
-        if self.getExtendedAttribute("Pref") and self.isExposedOffMainThread():
-            raise WebIDLError("[Pref] used on an interface member that is not "
-                              "main-thread-only",
-                              [self.location])
-
         if self.isAttr() or self.isMethod():
             if self.affects == "Everything" and self.dependsOn != "Everything":
                 raise WebIDLError("Interface member is flagged as affecting "