Implemented Houdini worklets.

34 files changed, 1209 insertions, 17 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 3610f0cf2cb..d0eec9a4440 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -196,6 +196,11 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "bitflags"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "bitflags"
 version = "0.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
@@ -2143,6 +2148,15 @@ dependencies = [
 ]
 
 [[package]]
+name = "pulldown-cmark"
+version = "0.0.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "bitflags 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "getopts 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "quasi"
 version = "0.32.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -2351,6 +2365,7 @@ dependencies = [
  "smallvec 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "style 0.0.1",
  "style_traits 0.0.1",
+ "swapper 0.0.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "time 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)",
  "tinyfiledialogs 2.5.9 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicode-segmentation 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2407,7 +2422,6 @@ dependencies = [
 name = "script_traits"
 version = "0.0.1"
 dependencies = [
- "app_units 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "bluetooth_traits 0.0.1",
  "canvas_traits 0.0.1",
  "cookie 0.6.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2739,6 +2753,15 @@ name = "size_of_test"
 version = "0.0.1"
 
 [[package]]
+name = "skeptic"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "pulldown-cmark 0.0.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "tempdir 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "slab"
 version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -2883,6 +2906,14 @@ dependencies = [
 ]
 
 [[package]]
+name = "swapper"
+version = "0.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "skeptic 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "syn"
 version = "0.11.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -2969,6 +3000,14 @@ dependencies = [
 ]
 
 [[package]]
+name = "tempdir"
+version = "0.3.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "rand 0.3.15 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "tendril"
 version = "0.2.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -3395,6 +3434,7 @@ dependencies = [
 "checksum bindgen 0.25.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ccaf8958532d7e570e905266ee2dc1094c3e5c3c3cfc2c299368747a30a5e654"
 "checksum bit-set 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "d9bf6104718e80d7b26a68fdbacff3481cfc05df670821affc7e9cbc1884400c"
 "checksum bit-vec 0.4.3 (registry+https://github.com/rust-lang/crates.io-index)" = "5b97c2c8e8bbb4251754f559df8af22fb264853c7d009084a576cdf12565089d"
+"checksum bitflags 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)" = "4f67931368edf3a9a51d29886d245f1c3db2f1ef0dcc9e35ff70341b78c10d23"
 "checksum bitflags 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "aad18937a628ec6abcd26d1489012cc0e18c21798210f491af69ded9b881106d"
 "checksum bitflags 0.8.2 (registry+https://github.com/rust-lang/crates.io-index)" = "1370e9fc2a6ae53aea8b7a5110edbd08836ed87c88736dfabccade1c2b44bff4"
 "checksum bitreader 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "80b13e2ab064ff3aa0bdbf1eff533f9822dc37899821f5f98c67f263eab51707"
@@ -3550,6 +3590,7 @@ dependencies = [
 "checksum png 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)" = "3cb773e9a557edb568ce9935cf783e3cdcabe06a9449d41b3e5506d88e582c82"
 "checksum precomputed-hash 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "cdf1fc3616b3ef726a847f2cd2388c646ef6a1f1ba4835c2629004da48184150"
 "checksum procedural-masquerade 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "9f566249236c6ca4340f7ca78968271f0ed2b0f234007a61b66f9ecd0af09260"
+"checksum pulldown-cmark 0.0.8 (registry+https://github.com/rust-lang/crates.io-index)" = "1058d7bb927ca067656537eec4e02c2b4b70eaaa129664c5b90c111e20326f41"
 "checksum quasi 0.32.0 (registry+https://github.com/rust-lang/crates.io-index)" = "18c45c4854d6d1cf5d531db97c75880feb91c958b0720f4ec1057135fec358b3"
 "checksum quasi_codegen 0.32.0 (registry+https://github.com/rust-lang/crates.io-index)" = "51b9e25fa23c044c1803f43ca59c98dac608976dd04ce799411edd58ece776d4"
 "checksum quote 0.3.15 (registry+https://github.com/rust-lang/crates.io-index)" = "7a6e920b65c65f10b2ae65c831a81a073a89edd28c7cce89475bff467ab4167a"
@@ -3586,12 +3627,14 @@ dependencies = [
 "checksum signpost 0.1.0 (git+https://github.com/pcwalton/signpost.git)" = "<none>"
 "checksum simd 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "7a94d14a2ae1f1f110937de5fb69e494372560181c7e1739a097fcc2cee37ba0"
 "checksum siphasher 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "0df90a788073e8d0235a67e50441d47db7c8ad9debd91cbf43736a2a92d36537"
+"checksum skeptic 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "dd7d8dc1315094150052d0ab767840376335a98ac66ef313ff911cdf439a5b69"
 "checksum slab 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "17b4fcaed89ab08ef143da37bc52adbcc04d4a69014f4c1208d6b51f0c47bc23"
 "checksum smallvec 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "4f8266519bc1d17d0b5b16f6c21295625d562841c708f6376f49028a43e9c11e"
 "checksum string_cache 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)" = "f55fba06c5e294108f22e8512eb598cb13388a117991e411a8df8f41a1219a75"
 "checksum string_cache_codegen 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "479cde50c3539481f33906a387f2bd17c8e87cb848c35b6021d41fb81ff9b4d7"
 "checksum string_cache_shared 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "b1884d1bc09741d466d9b14e6d37ac89d6909cbcac41dd9ae982d4d063bbedfc"
 "checksum strsim 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)" = "b4d15c810519a91cf877e7e36e63fe068815c678181439f2f29e2562147c3694"
+"checksum swapper 0.0.4 (registry+https://github.com/rust-lang/crates.io-index)" = "3ca610b32bb8bfc5e7f705480c3a1edfeb70b6582495d343872c8bee0dcf758c"
 "checksum syn 0.11.11 (registry+https://github.com/rust-lang/crates.io-index)" = "d3b891b9015c88c576343b9b3e41c2c11a51c219ef067b264bd9c8aa9b441dad"
 "checksum synom 0.11.3 (registry+https://github.com/rust-lang/crates.io-index)" = "a393066ed9010ebaed60b9eafa373d4b1baac186dd7e008555b0f702b51945b6"
 "checksum synstructure 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)" = "5ccc9780bf1aa601943988c2876ab22413c01ad1739689aa6af18d0aa0b3f38b"
@@ -3600,6 +3643,7 @@ dependencies = [
 "checksum syntex_pos 0.58.1 (registry+https://github.com/rust-lang/crates.io-index)" = "13ad4762fe52abc9f4008e85c4fb1b1fe3aa91ccb99ff4826a439c7c598e1047"
 "checksum syntex_syntax 0.58.1 (registry+https://github.com/rust-lang/crates.io-index)" = "6e0e4dbae163dd98989464c23dd503161b338790640e11537686f2ef0f25c791"
 "checksum target_build_utils 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "f42dc058080c19c6a58bdd1bf962904ee4f5ef1fe2a81b529f31dacc750c679f"
+"checksum tempdir 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)" = "87974a6f5c1dfb344d733055601650059a3363de2a6104819293baff662132d6"
 "checksum tendril 0.2.4 (registry+https://github.com/rust-lang/crates.io-index)" = "4ce04c250d202db8004921e3d3bc95eaa4f2126c6937a428ae39d12d0e38df62"
 "checksum term 0.4.5 (registry+https://github.com/rust-lang/crates.io-index)" = "d168af3930b369cfe245132550579d47dfd873d69470755a19c2c6568dbbd989"
 "checksum term_size 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "07b6c1ac5b3fffd75073276bca1ceed01f67a28537097a2a9539e116e50fb21a"
diff --git a/components/profile/time.rs b/components/profile/time.rs
index 928eca4a263..dfe5c37068d 100644
--- a/components/profile/time.rs
+++ b/components/profile/time.rs
@@ -150,6 +150,7 @@ impl Formattable for ProfilerCategory {
             ProfilerCategory::ScriptEnterFullscreen => "Script Enter Fullscreen",
             ProfilerCategory::ScriptExitFullscreen => "Script Exit Fullscreen",
             ProfilerCategory::ScriptWebVREvent => "Script WebVR Event",
+            ProfilerCategory::ScriptWorkletEvent => "Script Worklet Event",
             ProfilerCategory::ApplicationHeartbeat => "Application Heartbeat",
         };
         format!("{}{}", padding, name)
diff --git a/components/profile_traits/time.rs b/components/profile_traits/time.rs
index 7bbebf5f465..369c2fa366d 100644
--- a/components/profile_traits/time.rs
+++ b/components/profile_traits/time.rs
@@ -89,6 +89,7 @@ pub enum ProfilerCategory {
     ScriptEnterFullscreen = 0x77,
     ScriptExitFullscreen = 0x78,
     ScriptWebVREvent = 0x79,
+    ScriptWorkletEvent = 0x7a,
     ApplicationHeartbeat = 0x90,
 }
 
diff --git a/components/script/Cargo.toml b/components/script/Cargo.toml
index 306dd2a748d..388801aa011 100644
--- a/components/script/Cargo.toml
+++ b/components/script/Cargo.toml
@@ -83,6 +83,7 @@ servo_url = {path = "../url"}
 smallvec = "0.3"
 style = {path = "../style"}
 style_traits = {path = "../style_traits"}
+swapper = "0.0.4"
 time = "0.1.12"
 unicode-segmentation = "1.1.0"
 url = {version = "1.2", features = ["heap_size", "query_encoding"]}
diff --git a/components/script/dom/bindings/refcounted.rs b/components/script/dom/bindings/refcounted.rs
index dcf11626543..f22074d5fef 100644
--- a/components/script/dom/bindings/refcounted.rs
+++ b/components/script/dom/bindings/refcounted.rs
@@ -23,12 +23,17 @@
 //! as JS roots.
 
 use core::nonzero::NonZero;
+use dom::bindings::conversions::ToJSValConvertible;
+use dom::bindings::error::Error;
 use dom::bindings::js::Root;
 use dom::bindings::reflector::{DomObject, Reflector};
 use dom::bindings::trace::trace_reflector;
 use dom::promise::Promise;
+use js::jsapi::JSAutoCompartment;
 use js::jsapi::JSTracer;
 use libc;
+use script_thread::Runnable;
+use script_thread::ScriptThread;
 use std::cell::RefCell;
 use std::collections::hash_map::Entry::{Occupied, Vacant};
 use std::collections::hash_map::HashMap;
@@ -115,6 +120,40 @@ impl TrustedPromise {
             promise
         })
     }
+
+    /// A runnable which will reject the promise.
+    #[allow(unrooted_must_root)]
+    pub fn reject_runnable(self, error: Error) -> impl Runnable + Send {
+        struct RejectPromise(TrustedPromise, Error);
+        impl Runnable for RejectPromise {
+            fn main_thread_handler(self: Box<Self>, script_thread: &ScriptThread) {
+                let this = *self;
+                let cx = script_thread.get_cx();
+                let promise = this.0.root();
+                let _ac = JSAutoCompartment::new(cx, promise.reflector().get_jsobject().get());
+                promise.reject_error(cx, this.1);
+            }
+        }
+        RejectPromise(self, error)
+    }
+
+    /// A runnable which will resolve the promise.
+    #[allow(unrooted_must_root)]
+    pub fn resolve_runnable<T>(self, value: T) -> impl Runnable + Send where
+        T: ToJSValConvertible + Send
+    {
+        struct ResolvePromise<T>(TrustedPromise, T);
+        impl<T: ToJSValConvertible> Runnable for ResolvePromise<T> {
+            fn main_thread_handler(self: Box<Self>, script_thread: &ScriptThread) {
+                let this = *self;
+                let cx = script_thread.get_cx();
+                let promise = this.0.root();
+                let _ac = JSAutoCompartment::new(cx, promise.reflector().get_jsobject().get());
+                promise.resolve_native(cx, &this.1);
+            }
+        }
+        ResolvePromise(self, value)
+    }
 }
 
 /// A safe wrapper around a raw pointer to a DOM object that can be
diff --git a/components/script/dom/globalscope.rs b/components/script/dom/globalscope.rs
index 931d1f576aa..ce81958a2cb 100644
--- a/components/script/dom/globalscope.rs
+++ b/components/script/dom/globalscope.rs
@@ -19,6 +19,7 @@ use dom::event::{Event, EventBubbles, EventCancelable, EventStatus};
 use dom::eventtarget::EventTarget;
 use dom::window::Window;
 use dom::workerglobalscope::WorkerGlobalScope;
+use dom::workletglobalscope::WorkletGlobalScope;
 use dom_struct::dom_struct;
 use ipc_channel::ipc::IpcSender;
 use js::{JSCLASS_IS_DOMJSCLASS, JSCLASS_IS_GLOBAL};
@@ -259,6 +260,10 @@ impl GlobalScope {
             // https://html.spec.whatwg.org/multipage/#script-settings-for-workers:api-base-url
             return worker.get_url().clone();
         }
+        if let Some(worker) = self.downcast::<WorkletGlobalScope>() {
+            // https://drafts.css-houdini.org/worklets/#script-settings-for-worklets
+            return worker.base_url();
+        }
         unreachable!();
     }
 
@@ -270,6 +275,10 @@ impl GlobalScope {
         if let Some(worker) = self.downcast::<WorkerGlobalScope>() {
             return worker.get_url().clone();
         }
+        if let Some(worker) = self.downcast::<WorkletGlobalScope>() {
+            // TODO: is this the right URL to return?
+            return worker.base_url();
+        }
         unreachable!();
     }
 
@@ -349,14 +358,14 @@ impl GlobalScope {
 
     /// Evaluate JS code on this global scope.
     pub fn evaluate_js_on_global_with_result(
-            &self, code: &str, rval: MutableHandleValue) {
+            &self, code: &str, rval: MutableHandleValue) -> bool {
         self.evaluate_script_on_global_with_result(code, "", rval, 1)
     }
 
     /// Evaluate a JS script on this global scope.
     #[allow(unsafe_code)]
     pub fn evaluate_script_on_global_with_result(
-            &self, code: &str, filename: &str, rval: MutableHandleValue, line_number: u32) {
+            &self, code: &str, filename: &str, rval: MutableHandleValue, line_number: u32) -> bool {
         let metadata = time::TimerMetadata {
             url: if filename.is_empty() {
                 self.get_url().as_str().into()
@@ -379,16 +388,21 @@ impl GlobalScope {
                 let _ac = JSAutoCompartment::new(cx, globalhandle.get());
                 let _aes = AutoEntryScript::new(self);
                 let options = CompileOptionsWrapper::new(cx, filename.as_ptr(), line_number);
-                unsafe {
-                    if !Evaluate2(cx, options.ptr, code.as_ptr(),
-                                  code.len() as libc::size_t,
-                                  rval) {
-                        debug!("error evaluating JS string");
-                        report_pending_exception(cx, true);
-                    }
+
+                debug!("evaluating JS string");
+                let result = unsafe {
+                    Evaluate2(cx, options.ptr, code.as_ptr(),
+                              code.len() as libc::size_t,
+                              rval)
+                };
+
+                if !result {
+                    debug!("error evaluating JS string");
+                    unsafe { report_pending_exception(cx, true) };
                 }
 
                 maybe_resume_unwind();
+                result
             }
         )
     }
@@ -468,6 +482,9 @@ impl GlobalScope {
         if let Some(worker) = self.downcast::<WorkerGlobalScope>() {
             return worker.perform_a_microtask_checkpoint();
         }
+        if let Some(worker) = self.downcast::<WorkletGlobalScope>() {
+            return worker.perform_a_microtask_checkpoint();
+        }
         unreachable!();
     }
 
@@ -479,6 +496,9 @@ impl GlobalScope {
         if let Some(worker) = self.downcast::<WorkerGlobalScope>() {
             return worker.enqueue_microtask(job);
         }
+        if let Some(worker) = self.downcast::<WorkletGlobalScope>() {
+            return worker.enqueue_microtask(job);
+        }
         unreachable!();
     }
 
diff --git a/components/script/dom/mod.rs b/components/script/dom/mod.rs
index d155b8b6a47..62bffbdcbcd 100644
--- a/components/script/dom/mod.rs
+++ b/components/script/dom/mod.rs
@@ -423,6 +423,8 @@ pub mod testbindingiterable;
 pub mod testbindingpairiterable;
 pub mod testbindingproxy;
 pub mod testrunner;
+pub mod testworklet;
+pub mod testworkletglobalscope;
 pub mod text;
 pub mod textdecoder;
 pub mod textencoder;
@@ -469,6 +471,8 @@ pub mod worker;
 pub mod workerglobalscope;
 pub mod workerlocation;
 pub mod workernavigator;
+pub mod worklet;
+pub mod workletglobalscope;
 pub mod xmldocument;
 pub mod xmlhttprequest;
 pub mod xmlhttprequesteventtarget;
diff --git a/components/script/dom/promise.rs b/components/script/dom/promise.rs
index 13689023462..5595e999c9d 100644
--- a/components/script/dom/promise.rs
+++ b/components/script/dom/promise.rs
@@ -296,3 +296,4 @@ fn create_native_handler_function(cx: *mut JSContext,
         obj.get()
     }
 }
+
diff --git a/components/script/dom/testworklet.rs b/components/script/dom/testworklet.rs
new file mode 100644
index 00000000000..ea032e66faa
--- /dev/null
+++ b/components/script/dom/testworklet.rs
@@ -0,0 +1,61 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// check-tidy: no specs after this line
+
+use dom::bindings::codegen::Bindings::TestWorkletBinding::TestWorkletMethods;
+use dom::bindings::codegen::Bindings::TestWorkletBinding::Wrap;
+use dom::bindings::codegen::Bindings::WorkletBinding::WorkletBinding::WorkletMethods;
+use dom::bindings::codegen::Bindings::WorkletBinding::WorkletOptions;
+use dom::bindings::error::Fallible;
+use dom::bindings::js::JS;
+use dom::bindings::js::Root;
+use dom::bindings::reflector::Reflector;
+use dom::bindings::reflector::reflect_dom_object;
+use dom::bindings::str::DOMString;
+use dom::bindings::str::USVString;
+use dom::promise::Promise;
+use dom::window::Window;
+use dom::worklet::Worklet;
+use dom::workletglobalscope::WorkletGlobalScopeType;
+use dom_struct::dom_struct;
+use script_thread::ScriptThread;
+use std::rc::Rc;
+
+#[dom_struct]
+pub struct TestWorklet {
+    reflector: Reflector,
+    worklet: JS<Worklet>,
+}
+
+impl TestWorklet {
+    fn new_inherited(worklet: &Worklet) -> TestWorklet {
+        TestWorklet {
+            reflector: Reflector::new(),
+            worklet: JS::from_ref(worklet),
+        }
+    }
+
+    fn new(window: &Window) -> Root<TestWorklet> {
+        let worklet = Worklet::new(window, WorkletGlobalScopeType::Test);
+        reflect_dom_object(box TestWorklet::new_inherited(&*worklet), window, Wrap)
+    }
+
+    pub fn Constructor(window: &Window) -> Fallible<Root<TestWorklet>> {
+        Ok(TestWorklet::new(window))
+    }
+}
+
+impl TestWorkletMethods for TestWorklet {
+    #[allow(unrooted_must_root)]
+    fn AddModule(&self, moduleURL: USVString, options: &WorkletOptions) -> Rc<Promise> {
+        self.worklet.AddModule(moduleURL, options)
+    }
+
+    fn Lookup(&self, key: DOMString) -> Option<DOMString> {
+        let id = self.worklet.worklet_id();
+        let pool = ScriptThread::worklet_thread_pool();
+        pool.test_worklet_lookup(id, String::from(key)).map(DOMString::from)
+    }
+}
diff --git a/components/script/dom/testworkletglobalscope.rs b/components/script/dom/testworkletglobalscope.rs
new file mode 100644
index 00000000000..dfd000ac5c1
--- /dev/null
+++ b/components/script/dom/testworkletglobalscope.rs
@@ -0,0 +1,66 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use dom::bindings::cell::DOMRefCell;
+use dom::bindings::codegen::Bindings::TestWorkletGlobalScopeBinding;
+use dom::bindings::codegen::Bindings::TestWorkletGlobalScopeBinding::TestWorkletGlobalScopeMethods;
+use dom::bindings::js::Root;
+use dom::bindings::str::DOMString;
+use dom::workletglobalscope::WorkletGlobalScope;
+use dom::workletglobalscope::WorkletGlobalScopeInit;
+use dom_struct::dom_struct;
+use js::rust::Runtime;
+use msg::constellation_msg::PipelineId;
+use servo_url::ServoUrl;
+use std::collections::HashMap;
+use std::sync::mpsc::Sender;
+
+// check-tidy: no specs after this line
+
+#[dom_struct]
+pub struct TestWorkletGlobalScope {
+    // The worklet global for this object
+    worklet_global: WorkletGlobalScope,
+    // The key/value pairs
+    lookup_table: DOMRefCell<HashMap<String, String>>,
+}
+
+impl TestWorkletGlobalScope {
+    #[allow(unsafe_code)]
+    pub fn new(runtime: &Runtime,
+               pipeline_id: PipelineId,
+               base_url: ServoUrl,
+               init: &WorkletGlobalScopeInit)
+               -> Root<TestWorkletGlobalScope>
+    {
+        debug!("Creating test worklet global scope for pipeline {}.", pipeline_id);
+        let global = box TestWorkletGlobalScope {
+            worklet_global: WorkletGlobalScope::new_inherited(pipeline_id, base_url, init),
+            lookup_table: Default::default(),
+        };
+        unsafe { TestWorkletGlobalScopeBinding::Wrap(runtime.cx(), global) }
+    }
+
+    pub fn perform_a_worklet_task(&self, task: TestWorkletTask) {
+        match task {
+            TestWorkletTask::Lookup(key, sender) => {
+                debug!("Looking up key {}.", key);
+                let result = self.lookup_table.borrow().get(&key).cloned();
+                let _ = sender.send(result);
+            }
+        }
+    }
+}
+
+impl TestWorkletGlobalScopeMethods for TestWorkletGlobalScope {
+    fn RegisterKeyValue(&self, key: DOMString, value: DOMString) {
+        debug!("Registering test worklet key/value {}/{}.", key, value);
+        self.lookup_table.borrow_mut().insert(String::from(key), String::from(value));
+    }
+}
+
+/// Tasks which can be performed by test worklets.
+pub enum TestWorkletTask {
+    Lookup(String, Sender<Option<String>>),
+}
diff --git a/components/script/dom/webidls/Console.webidl b/components/script/dom/webidls/Console.webidl
index 90f9bb9f58e..7c4c6906a27 100644
--- a/components/script/dom/webidls/Console.webidl
+++ b/components/script/dom/webidls/Console.webidl
@@ -10,7 +10,7 @@
  */
 
 [ClassString="Console",
- Exposed=(Window,Worker),
+ Exposed=(Window,Worker,Worklet),
  ProtoObjectHack]
 namespace console {
   // These should be DOMString message, DOMString message2, ...
diff --git a/components/script/dom/webidls/EventTarget.webidl b/components/script/dom/webidls/EventTarget.webidl
index ee6e5d722a8..ad25712122a 100644
--- a/components/script/dom/webidls/EventTarget.webidl
+++ b/components/script/dom/webidls/EventTarget.webidl
@@ -5,7 +5,7 @@
  * https://dom.spec.whatwg.org/#interface-eventtarget
  */
 
-[Abstract, Exposed=(Window,Worker)]
+[Abstract, Exposed=(Window,Worker,Worklet)]
 interface EventTarget {
   void addEventListener(DOMString type,
                         EventListener? listener,
diff --git a/components/script/dom/webidls/GlobalScope.webidl b/components/script/dom/webidls/GlobalScope.webidl
index 7dab4f3afa7..2681d236dbc 100644
--- a/components/script/dom/webidls/GlobalScope.webidl
+++ b/components/script/dom/webidls/GlobalScope.webidl
@@ -5,6 +5,6 @@
 // This interface is entirely internal to Servo, and should not be accessible to
 // web pages.
 
-[Exposed=(Window,Worker),
+[Exposed=(Window,Worker,Worklet),
  Inline]
 interface GlobalScope : EventTarget {};
diff --git a/components/script/dom/webidls/TestWorklet.webidl b/components/script/dom/webidls/TestWorklet.webidl
new file mode 100644
index 00000000000..c1f1965a1e0
--- /dev/null
+++ b/components/script/dom/webidls/TestWorklet.webidl
@@ -0,0 +1,12 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+[Pref="dom.worklet.testing.enabled", Exposed=(Window), Constructor]
+interface TestWorklet {
+   [NewObject] Promise<void> 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
new file mode 100644
index 00000000000..44027ab8dc6
--- /dev/null
+++ b/components/script/dom/webidls/TestWorkletGlobalScope.webidl
@@ -0,0 +1,11 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+[Global=(Worklet,TestWorklet), Exposed=TestWorklet]
+interface TestWorkletGlobalScope : WorkletGlobalScope {
+    void registerKeyValue(DOMString key, DOMString value);
+};
diff --git a/components/script/dom/webidls/VoidFunction.webidl b/components/script/dom/webidls/VoidFunction.webidl
new file mode 100644
index 00000000000..82d4a666c51
--- /dev/null
+++ b/components/script/dom/webidls/VoidFunction.webidl
@@ -0,0 +1,13 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+/*
+ * The origin of this IDL file is
+ * https://heycam.github.io/webidl/#VoidFunction
+ *
+ * © Copyright 2004-2011 Apple Computer, Inc., Mozilla Foundation, and
+ * Opera Software ASA. You are granted a license to use, reproduce
+ * and create derivative works of this document.
+ */
+
+callback VoidFunction = void ();
diff --git a/components/script/dom/webidls/Window.webidl b/components/script/dom/webidls/Window.webidl
index 47c753f43b1..548821ac971 100644
--- a/components/script/dom/webidls/Window.webidl
+++ b/components/script/dom/webidls/Window.webidl
@@ -201,3 +201,4 @@ partial interface Window {
    readonly attribute TestRunner testRunner;
    //readonly attribute EventSender eventSender;
 };
+
diff --git a/components/script/dom/webidls/Worklet.webidl b/components/script/dom/webidls/Worklet.webidl
new file mode 100644
index 00000000000..5bb39bebd96
--- /dev/null
+++ b/components/script/dom/webidls/Worklet.webidl
@@ -0,0 +1,13 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// https://drafts.css-houdini.org/worklets/#worklet
+[Exposed=(Window)]
+interface Worklet {
+    [NewObject] Promise<void> addModule(USVString moduleURL, optional WorkletOptions options);
+};
+
+dictionary WorkletOptions {
+    RequestCredentials credentials = "omit";
+};
diff --git a/components/script/dom/webidls/WorkletGlobalScope.webidl b/components/script/dom/webidls/WorkletGlobalScope.webidl
new file mode 100644
index 00000000000..ca29296a10e
--- /dev/null
+++ b/components/script/dom/webidls/WorkletGlobalScope.webidl
@@ -0,0 +1,10 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// https://drafts.css-houdini.org/worklets/#workletglobalscope
+// TODO: The spec IDL doesn't make this a subclass of EventTarget
+//       https://github.com/whatwg/html/issues/2611
+[Exposed=Worklet]
+interface WorkletGlobalScope: GlobalScope {
+};
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 65d1a591adc..097f136e29c 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -49,6 +49,7 @@ use dom::screen::Screen;
 use dom::storage::Storage;
 use dom::testrunner::TestRunner;
 use dom::windowproxy::WindowProxy;
+use dom::worklet::Worklet;
 use dom_struct::dom_struct;
 use euclid::{Point2D, Rect, Size2D};
 use fetch;
@@ -273,6 +274,9 @@ pub struct Window {
     /// Directory to store unminified scripts for this window if unminify-js
     /// opt is enabled.
     unminified_js_dir: DOMRefCell<Option<String>>,
+
+    /// Worklets
+    test_worklet: MutNullableJS<Worklet>,
 }
 
 impl Window {
@@ -1830,6 +1834,7 @@ impl Window {
             permission_state_invocation_results: DOMRefCell::new(HashMap::new()),
             pending_layout_images: DOMRefCell::new(HashMap::new()),
             unminified_js_dir: DOMRefCell::new(None),
+            test_worklet: Default::default(),
         };
 
         unsafe {
diff --git a/components/script/dom/worklet.rs b/components/script/dom/worklet.rs
new file mode 100644
index 00000000000..fa5b3950b51
--- /dev/null
+++ b/components/script/dom/worklet.rs
@@ -0,0 +1,637 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! An implementation of Houdini worklets.
+//!
+//! The goal of this implementation is to maximize responsiveness of worklets,
+//! and in particular to ensure that the thread performing worklet tasks
+//! is never busy GCing or loading worklet code. We do this by providing a custom
+//! thread pool implementation, which only performs GC or code loading on
+//! a backup thread, not on the primary worklet thread.
+
+use dom::bindings::codegen::Bindings::RequestBinding::RequestCredentials;
+use dom::bindings::codegen::Bindings::WindowBinding::WindowBinding::WindowMethods;
+use dom::bindings::codegen::Bindings::WorkletBinding::WorkletMethods;
+use dom::bindings::codegen::Bindings::WorkletBinding::WorkletOptions;
+use dom::bindings::codegen::Bindings::WorkletBinding::Wrap;
+use dom::bindings::error::Error;
+use dom::bindings::inheritance::Castable;
+use dom::bindings::js::JS;
+use dom::bindings::js::Root;
+use dom::bindings::js::RootCollection;
+use dom::bindings::refcounted::TrustedPromise;
+use dom::bindings::reflector::Reflector;
+use dom::bindings::reflector::reflect_dom_object;
+use dom::bindings::str::USVString;
+use dom::bindings::trace::JSTraceable;
+use dom::bindings::trace::RootedTraceableBox;
+use dom::globalscope::GlobalScope;
+use dom::promise::Promise;
+use dom::testworkletglobalscope::TestWorkletTask;
+use dom::window::Window;
+use dom::workletglobalscope::WorkletGlobalScope;
+use dom::workletglobalscope::WorkletGlobalScopeInit;
+use dom::workletglobalscope::WorkletGlobalScopeType;
+use dom::workletglobalscope::WorkletTask;
+use dom_struct::dom_struct;
+use js::jsapi::JSGCParamKey;
+use js::jsapi::JSTracer;
+use js::jsapi::JS_GC;
+use js::jsapi::JS_GetGCParameter;
+use js::rust::Runtime;
+use msg::constellation_msg::PipelineId;
+use net_traits::IpcSend;
+use net_traits::load_whole_resource;
+use net_traits::request::Destination;
+use net_traits::request::RequestInit;
+use net_traits::request::RequestMode;
+use net_traits::request::Type as RequestType;
+use script_runtime::CommonScriptMsg;
+use script_runtime::ScriptThreadEventCategory;
+use script_runtime::StackRootTLS;
+use script_runtime::new_rt_and_cx;
+use script_thread::MainThreadScriptMsg;
+use script_thread::Runnable;
+use script_thread::ScriptThread;
+use servo_rand;
+use servo_url::ImmutableOrigin;
+use servo_url::ServoUrl;
+use std::cmp::max;
+use std::collections::HashMap;
+use std::collections::hash_map;
+use std::rc::Rc;
+use std::sync::Arc;
+use std::sync::atomic::AtomicIsize;
+use std::sync::atomic::Ordering;
+use std::sync::mpsc;
+use std::sync::mpsc::Receiver;
+use std::sync::mpsc::Sender;
+use std::thread;
+use style::thread_state;
+use swapper::Swapper;
+use swapper::swapper;
+use uuid::Uuid;
+
+// Magic numbers
+const WORKLET_THREAD_POOL_SIZE: u32 = 3;
+const MIN_GC_THRESHOLD: u32 = 1_000_000;
+
+#[dom_struct]
+/// https://drafts.css-houdini.org/worklets/#worklet
+pub struct Worklet {
+    reflector: Reflector,
+    window: JS<Window>,
+    worklet_id: WorkletId,
+    global_type: WorkletGlobalScopeType,
+}
+
+impl Worklet {
+    fn new_inherited(window: &Window, global_type: WorkletGlobalScopeType) -> Worklet {
+        Worklet {
+            reflector: Reflector::new(),
+            window: JS::from_ref(window),
+            worklet_id: WorkletId::new(),
+            global_type: global_type,
+        }
+    }
+
+    pub fn new(window: &Window, global_type: WorkletGlobalScopeType) -> Root<Worklet> {
+        debug!("Creating worklet {:?}.", global_type);
+        reflect_dom_object(box Worklet::new_inherited(window, global_type), window, Wrap)
+    }
+
+    pub fn worklet_id(&self) -> WorkletId {
+        self.worklet_id
+    }
+
+    #[allow(dead_code)]
+    pub fn worklet_global_scope_type(&self) -> WorkletGlobalScopeType {
+        self.global_type
+    }
+}
+
+impl WorkletMethods for Worklet {
+    #[allow(unrooted_must_root)]
+    /// https://drafts.css-houdini.org/worklets/#dom-worklet-addmodule
+    fn AddModule(&self, module_url: USVString, options: &WorkletOptions) -> Rc<Promise> {
+        // Step 1.
+        let promise = Promise::new(self.window.upcast());
+
+        // Step 3.
+        let module_url_record = match self.window.Document().base_url().join(&module_url.0) {
+            Ok(url) => url,
+            Err(err) => {
+                // Step 4.
+                debug!("URL {:?} parse error {:?}.", module_url.0, err);
+                promise.reject_error(self.window.get_cx(), Error::Syntax);
+                return promise;
+            }
+        };
+        debug!("Adding Worklet module {}.", module_url_record);
+
+        // Steps 6-12 in parallel.
+        let pending_tasks_struct = PendingTasksStruct::new();
+        let global = self.window.upcast::<GlobalScope>();
+        let pool = ScriptThread::worklet_thread_pool();
+
+        pool.fetch_and_invoke_a_worklet_script(global.pipeline_id(),
+                                               self.worklet_id,
+                                               self.global_type,
+                                               self.window.origin().immutable().clone(),
+                                               global.api_base_url(),
+                                               module_url_record,
+                                               options.credentials.clone(),
+                                               pending_tasks_struct,
+                                               &promise);
+
+        // Step 5.
+        promise
+    }
+}
+
+/// A guid for worklets.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, JSTraceable)]
+pub struct WorkletId(Uuid);
+
+known_heap_size!(0, WorkletId);
+
+impl WorkletId {
+    fn new() -> WorkletId {
+        WorkletId(servo_rand::random())
+    }
+}
+
+/// https://drafts.css-houdini.org/worklets/#pending-tasks-struct
+#[derive(Clone, Debug)]
+struct PendingTasksStruct(Arc<AtomicIsize>);
+
+impl PendingTasksStruct {
+    fn new() -> PendingTasksStruct {
+        PendingTasksStruct(Arc::new(AtomicIsize::new(WORKLET_THREAD_POOL_SIZE as isize)))
+    }
+
+    fn set_counter_to(&self, value: isize) -> isize {
+        self.0.swap(value, Ordering::AcqRel)
+    }
+
+    fn decrement_counter_by(&self, offset: isize) -> isize {
+        self.0.fetch_sub(offset, Ordering::AcqRel)
+    }
+}
+
+/// Worklets execute in a dedicated thread pool.
+///
+/// The goal is to ensure that there is a primary worklet thread,
+/// which is able to responsively execute worklet code. In particular,
+/// worklet execution should not be delayed by GC, or by script
+/// loading.
+///
+/// To achieve this, we implement a three-thread pool, with the
+/// threads cycling between three thread roles:
+///
+///  * The primary worklet thread is the one available to execute
+///    worklet code.
+///
+///  * The hot backup thread may peform GC, but otherwise is expected
+///    to take over the primary role.
+///
+///  * The cold backup thread may peform script loading and other
+///    long-running tasks.
+///
+/// In the implementation, we use two kinds of messages:
+///
+///  * Data messages are expected to be processed quickly, and include
+///    the worklet tasks to be performed by the primary thread, as
+///    well as requests to change role or quit execution.
+///
+///  * Control messages are expected to be processed more slowly, and
+///    include script loading.
+///
+/// Data messages are targeted at a role, for example, task execution
+/// is expected to be performed by whichever thread is currently
+/// primary. Control messages are targeted at a thread, for example
+/// adding a module is performed in every thread, even if they change roles
+/// in the middle of module loading.
+///
+/// The thread pool lives in the script thread, and is initialized
+/// when a worklet adds a module. It is dropped when the script thread
+/// is dropped, and asks each of the worklet threads to quit.
+
+#[derive(Clone, JSTraceable)]
+pub struct WorkletThreadPool {
+    // Channels to send data messages to the three roles.
+    primary_sender: Sender<WorkletData>,
+    hot_backup_sender: Sender<WorkletData>,
+    cold_backup_sender: Sender<WorkletData>,
+    // Channels to send control messages to the three threads.
+    control_sender_0: Sender<WorkletControl>,
+    control_sender_1: Sender<WorkletControl>,
+    control_sender_2: Sender<WorkletControl>,
+}
+
+impl Drop for WorkletThreadPool {
+    fn drop(&mut self) {
+        let _ = self.cold_backup_sender.send(WorkletData::Quit);
+        let _ = self.hot_backup_sender.send(WorkletData::Quit);
+        let _ = self.primary_sender.send(WorkletData::Quit);
+    }
+}
+
+impl WorkletThreadPool {
+    /// Create a new thread pool and spawn the threads.
+    /// When the thread pool is dropped, the threads will be asked to quit.
+    pub fn spawn(script_sender: Sender<MainThreadScriptMsg>, global_init: WorkletGlobalScopeInit) -> WorkletThreadPool {
+        let primary_role = WorkletThreadRole::new(false, false);
+        let hot_backup_role = WorkletThreadRole::new(true, false);
+        let cold_backup_role = WorkletThreadRole::new(false, true);
+        let primary_sender = primary_role.sender.clone();
+        let hot_backup_sender = hot_backup_role.sender.clone();
+        let cold_backup_sender = cold_backup_role.sender.clone();
+        let init = WorkletThreadInit {
+            hot_backup_sender: hot_backup_sender.clone(),
+            cold_backup_sender: cold_backup_sender.clone(),
+            script_sender: script_sender.clone(),
+            global_init: global_init,
+        };
+        WorkletThreadPool {
+            primary_sender: primary_sender,
+            hot_backup_sender: hot_backup_sender,
+            cold_backup_sender: cold_backup_sender,
+            control_sender_0: WorkletThread::spawn(primary_role, init.clone()),
+            control_sender_1: WorkletThread::spawn(hot_backup_role, init.clone()),
+            control_sender_2: WorkletThread::spawn(cold_backup_role, init),
+        }
+    }
+
+    /// Loads a worklet module into every worklet thread.
+    /// If all of the threads load successfully, the promise is resolved.
+    /// If any of the threads fails to load, the promise is rejected.
+    /// https://drafts.css-houdini.org/worklets/#fetch-and-invoke-a-worklet-script
+    fn fetch_and_invoke_a_worklet_script(&self,
+                                         pipeline_id: PipelineId,
+                                         worklet_id: WorkletId,
+                                         global_type: WorkletGlobalScopeType,
+                                         origin: ImmutableOrigin,
+                                         base_url: ServoUrl,
+                                         script_url: ServoUrl,
+                                         credentials: RequestCredentials,
+                                         pending_tasks_struct: PendingTasksStruct,
+                                         promise: &Rc<Promise>)
+    {
+        // Send each thread a control message asking it to load the script.
+        for sender in &[&self.control_sender_0, &self.control_sender_1, &self.control_sender_2] {
+            let _ = sender.send(WorkletControl::FetchAndInvokeAWorkletScript {
+                pipeline_id: pipeline_id,
+                worklet_id: worklet_id,
+                global_type: global_type,
+                origin: origin.clone(),
+                base_url: base_url.clone(),
+                script_url: script_url.clone(),
+                credentials: credentials,
+                pending_tasks_struct: pending_tasks_struct.clone(),
+                promise: TrustedPromise::new(promise.clone()),
+            });
+        }
+        // If any of the threads are blocked waiting on data, wake them up.
+        let _ = self.cold_backup_sender.send(WorkletData::WakeUp);
+        let _ = self.hot_backup_sender.send(WorkletData::WakeUp);
+        let _ = self.primary_sender.send(WorkletData::WakeUp);
+    }
+
+    /// For testing.
+    pub fn test_worklet_lookup(&self, id: WorkletId, key: String) -> Option<String> {
+        let (sender, receiver) = mpsc::channel();
+        let msg = WorkletData::Task(id, WorkletTask::Test(TestWorkletTask::Lookup(key, sender)));
+        let _ = self.primary_sender.send(msg);
+        receiver.recv().expect("Test worklet has died?")
+    }
+}
+
+/// The data messages sent to worklet threads
+enum WorkletData {
+    Task(WorkletId, WorkletTask),
+    StartSwapRoles(Sender<WorkletData>),
+    FinishSwapRoles(Swapper<WorkletThreadRole>),
+    WakeUp,
+    Quit,
+}
+
+/// The control message sent to worklet threads
+enum WorkletControl {
+    FetchAndInvokeAWorkletScript {
+        pipeline_id: PipelineId,
+        worklet_id: WorkletId,
+        global_type: WorkletGlobalScopeType,
+        origin: ImmutableOrigin,
+        base_url: ServoUrl,
+        script_url: ServoUrl,
+        credentials: RequestCredentials,
+        pending_tasks_struct: PendingTasksStruct,
+        promise: TrustedPromise,
+    },
+}
+
+/// A role that a worklet thread can be playing.
+///
+/// These roles are used as tokens or capabilities, we track unique
+/// ownership using Rust's types, and use atomic swapping to exchange
+/// them between worklet threads. This ensures that each thread pool has
+/// exactly one primary, one hot backup and one cold backup.
+struct WorkletThreadRole {
+    receiver: Receiver<WorkletData>,
+    sender: Sender<WorkletData>,
+    is_hot_backup: bool,
+    is_cold_backup: bool,
+}
+
+impl WorkletThreadRole {
+    fn new(is_hot_backup: bool, is_cold_backup: bool) -> WorkletThreadRole {
+        let (sender, receiver) = mpsc::channel();
+        WorkletThreadRole {
+            sender: sender,
+            receiver: receiver,
+            is_hot_backup: is_hot_backup,
+            is_cold_backup: is_cold_backup,
+        }
+    }
+}
+
+/// Data to initialize a worklet thread.
+#[derive(Clone)]
+struct WorkletThreadInit {
+    /// Senders
+    hot_backup_sender: Sender<WorkletData>,
+    cold_backup_sender: Sender<WorkletData>,
+    script_sender: Sender<MainThreadScriptMsg>,
+
+    /// Data for initializing new worklet global scopes
+    global_init: WorkletGlobalScopeInit,
+}
+
+/// A thread for executing worklets.
+#[must_root]
+struct WorkletThread {
+    /// Which role the thread is currently playing
+    role: WorkletThreadRole,
+
+    /// The thread's receiver for control messages
+    control_receiver: Receiver<WorkletControl>,
+
+    /// Senders
+    hot_backup_sender: Sender<WorkletData>,
+    cold_backup_sender: Sender<WorkletData>,
+    script_sender: Sender<MainThreadScriptMsg>,
+
+    /// Data for initializing new worklet global scopes
+    global_init: WorkletGlobalScopeInit,
+
+    /// The global scopes created by this thread
+    global_scopes: HashMap<WorkletId, JS<WorkletGlobalScope>>,
+
+    /// A one-place buffer for control messages
+    control_buffer: Option<WorkletControl>,
+
+    /// The JS runtime
+    runtime: Runtime,
+    should_gc: bool,
+    gc_threshold: u32,
+}
+
+#[allow(unsafe_code)]
+unsafe impl JSTraceable for WorkletThread {
+    unsafe fn trace(&self, trc: *mut JSTracer) {
+        debug!("Tracing worklet thread.");
+        self.global_scopes.trace(trc);
+    }
+}
+
+impl WorkletThread {
+    /// Spawn a new worklet thread, returning the channel to send it control messages.
+    #[allow(unsafe_code)]
+    #[allow(unrooted_must_root)]
+    fn spawn(role: WorkletThreadRole, init: WorkletThreadInit) -> Sender<WorkletControl> {
+        let (control_sender, control_receiver) = mpsc::channel();
+        // TODO: name this thread
+        thread::spawn(move || {
+            // TODO: add a new IN_WORKLET thread state?
+            // TODO: set interrupt handler?
+            // TODO: configure the JS runtime (e.g. discourage GC, encourage agressive JIT)
+            debug!("Initializing worklet thread.");
+            thread_state::initialize(thread_state::SCRIPT | thread_state::IN_WORKER);
+            let roots = RootCollection::new();
+            let _stack_roots_tls = StackRootTLS::new(&roots);
+            let mut thread = RootedTraceableBox::new(WorkletThread {
+                role: role,
+                control_receiver: control_receiver,
+                hot_backup_sender: init.hot_backup_sender,
+                cold_backup_sender: init.cold_backup_sender,
+                script_sender: init.script_sender,
+                global_init: init.global_init,
+                global_scopes: HashMap::new(),
+                control_buffer: None,
+                runtime: unsafe { new_rt_and_cx() },
+                should_gc: false,
+                gc_threshold: MIN_GC_THRESHOLD,
+            });
+            thread.run();
+        });
+        control_sender
+    }
+
+    /// The main event loop for a worklet thread
+    fn run(&mut self) {
+        loop {
+            // The handler for data messages
+            let message = self.role.receiver.recv().unwrap();
+            match message {
+                // The whole point of this thread pool is to perform tasks!
+                WorkletData::Task(id, task) => {
+                    self.perform_a_worklet_task(id, task);
+                }
+                // To start swapping roles, get ready to perform an atomic swap,
+                // and block waiting for the other end to finish it.
+                // NOTE: the cold backup can block on the primary or the hot backup;
+                //       the hot backup can block on the primary;
+                //       the primary can block on nothing;
+                //       this total ordering on thread roles is what guarantees deadlock-freedom.
+                WorkletData::StartSwapRoles(sender) => {
+                    let (our_swapper, their_swapper) = swapper();
+                    sender.send(WorkletData::FinishSwapRoles(their_swapper)).unwrap();
+                    let _ = our_swapper.swap(&mut self.role);
+                }
+                // To finish swapping roles, perform the atomic swap.
+                // The other end should have already started the swap, so this shouldn't block.
+                WorkletData::FinishSwapRoles(swapper) => {
+                    let _ = swapper.swap(&mut self.role);
+                }
+                // Wake up! There may be control messages to process.
+                WorkletData::WakeUp => {
+                }
+                // Quit!
+                WorkletData::Quit => {
+                    return;
+                }
+            }
+            // Only process control messages if we're the cold backup,
+            // otherwise if there are outstanding control messages,
+            // try to become the cold backup.
+            if self.role.is_cold_backup {
+                if let Some(control) = self.control_buffer.take() {
+                    self.process_control(control);
+                }
+                while let Ok(control) = self.control_receiver.try_recv() {
+                    self.process_control(control);
+                }
+                self.gc();
+            } else if self.control_buffer.is_none() {
+                if let Ok(control) = self.control_receiver.try_recv() {
+                    self.control_buffer = Some(control);
+                    let msg = WorkletData::StartSwapRoles(self.role.sender.clone());
+                    let _ = self.cold_backup_sender.send(msg);
+                }
+            }
+            // If we are tight on memory, and we're a backup then perform a gc.
+            // If we are tight on memory, and we're the primary then try to become the hot backup.
+            // Hopefully this happens soon!
+            if self.current_memory_usage() > self.gc_threshold {
+                if self.role.is_hot_backup || self.role.is_cold_backup {
+                    self.should_gc = false;
+                    self.gc();
+                } else if !self.should_gc {
+                    self.should_gc = true;
+                    let msg = WorkletData::StartSwapRoles(self.role.sender.clone());
+                    let _ = self.hot_backup_sender.send(msg);
+                }
+            }
+        }
+    }
+
+    /// The current memory usage of the thread
+    #[allow(unsafe_code)]
+    fn current_memory_usage(&self) -> u32 {
+        unsafe { JS_GetGCParameter(self.runtime.rt(), JSGCParamKey::JSGC_BYTES) }
+    }
+
+    /// Perform a GC.
+    #[allow(unsafe_code)]
+    fn gc(&mut self) {
+        debug!("BEGIN GC (usage = {}, threshold = {}).", self.current_memory_usage(), self.gc_threshold);
+        unsafe { JS_GC(self.runtime.rt()) };
+        self.gc_threshold = max(MIN_GC_THRESHOLD, self.current_memory_usage() * 2);
+        debug!("END GC (usage = {}, threshold = {}).", self.current_memory_usage(), self.gc_threshold);
+    }
+
+    /// Get the worklet global scope for a given worklet.
+    /// Creates the worklet global scope if it doesn't exist.
+    fn get_worklet_global_scope(&mut self,
+                                pipeline_id: PipelineId,
+                                worklet_id: WorkletId,
+                                global_type: WorkletGlobalScopeType,
+                                base_url: ServoUrl)
+                                -> Root<WorkletGlobalScope>
+    {
+        match self.global_scopes.entry(worklet_id) {
+            hash_map::Entry::Occupied(entry) => Root::from_ref(entry.get()),
+            hash_map::Entry::Vacant(entry) => {
+                let result = global_type.new(&self.runtime, pipeline_id, base_url, &self.global_init);
+                entry.insert(JS::from_ref(&*result));
+                result
+            },
+        }
+    }
+
+    /// Fetch and invoke a worklet script.
+    /// https://drafts.css-houdini.org/worklets/#fetch-and-invoke-a-worklet-script
+    fn fetch_and_invoke_a_worklet_script(&self,
+                                         global_scope: &WorkletGlobalScope,
+                                         origin: ImmutableOrigin,
+                                         script_url: ServoUrl,
+                                         credentials: RequestCredentials,
+                                         pending_tasks_struct: PendingTasksStruct,
+                                         promise: TrustedPromise)
+    {
+        debug!("Fetching from {}.", script_url);
+        // Step 1.
+        // TODO: Settings object?
+
+        // Step 2.
+        // TODO: Fetch a module graph, not just a single script.
+        // TODO: Fetch the script asynchronously?
+        // TODO: Caching.
+        // TODO: Avoid re-parsing the origin as a URL.
+        let resource_fetcher = self.global_init.resource_threads.sender();
+        let origin_url = ServoUrl::parse(&*origin.unicode_serialization()).expect("Failed to parse origin as URL.");
+        let request = RequestInit {
+            url: script_url,
+            type_: RequestType::Script,
+            destination: Destination::Script,
+            mode: RequestMode::CorsMode,
+            origin: origin_url,
+            credentials_mode: credentials.into(),
+            .. RequestInit::default()
+        };
+        let script = load_whole_resource(request, &resource_fetcher).ok()
+            .and_then(|(_, bytes)| String::from_utf8(bytes).ok());
+
+        // Step 4.
+        // NOTE: the spec parses and executes the script in separate steps,
+        // but our JS API doesn't separate these, so we do the steps out of order.
+        let ok = script.map(|script| global_scope.evaluate_js(&*script)).unwrap_or(false);
+
+        if !ok {
+            // Step 3.
+            debug!("Failed to load script.");
+            let old_counter = pending_tasks_struct.set_counter_to(-1);
+            if old_counter > 0 {
+                self.run_in_script_thread(promise.reject_runnable(Error::Abort));
+            }
+        } else {
+            // Step 5.
+            debug!("Finished adding script.");
+            let old_counter = pending_tasks_struct.decrement_counter_by(1);
+            if old_counter == 1 {
+                // TODO: trigger a reflow?
+                self.run_in_script_thread(promise.resolve_runnable(()));
+            }
+        }
+    }
+
+    /// Perform a task.
+    fn perform_a_worklet_task(&self, worklet_id: WorkletId, task: WorkletTask) {
+        match self.global_scopes.get(&worklet_id) {
+            Some(global) => global.perform_a_worklet_task(task),
+            None => return warn!("No such worklet as {:?}.", worklet_id),
+        }
+    }
+
+    /// Process a control message.
+    fn process_control(&mut self, control: WorkletControl) {
+        match control {
+            WorkletControl::FetchAndInvokeAWorkletScript {
+                pipeline_id, worklet_id, global_type, origin, base_url,
+                script_url, credentials, pending_tasks_struct, promise,
+            } => {
+                let global = self.get_worklet_global_scope(pipeline_id,
+                                                           worklet_id,
+                                                           global_type,
+                                                           base_url);
+                self.fetch_and_invoke_a_worklet_script(&*global,
+                                                       origin,
+                                                       script_url,
+                                                       credentials,
+                                                       pending_tasks_struct,
+                                                       promise)
+            }
+        }
+    }
+
+    /// Run a runnable in the main script thread.
+    fn run_in_script_thread<R>(&self, runnable: R) where
+        R: 'static + Send + Runnable,
+    {
+        let msg = CommonScriptMsg::RunnableMsg(ScriptThreadEventCategory::WorkletEvent, box runnable);
+        let msg = MainThreadScriptMsg::Common(msg);
+        self.script_sender.send(msg).expect("Worklet thread outlived script thread.");
+    }
+}
diff --git a/components/script/dom/workletglobalscope.rs b/components/script/dom/workletglobalscope.rs
new file mode 100644
index 00000000000..a2e2463ca27
--- /dev/null
+++ b/components/script/dom/workletglobalscope.rs
@@ -0,0 +1,143 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use devtools_traits::ScriptToDevtoolsControlMsg;
+use dom::bindings::inheritance::Castable;
+use dom::bindings::js::Root;
+use dom::globalscope::GlobalScope;
+use dom::testworkletglobalscope::TestWorkletGlobalScope;
+use dom::testworkletglobalscope::TestWorkletTask;
+use dom_struct::dom_struct;
+use ipc_channel::ipc;
+use ipc_channel::ipc::IpcSender;
+use js::jsval::UndefinedValue;
+use js::rust::Runtime;
+use microtask::Microtask;
+use microtask::MicrotaskQueue;
+use msg::constellation_msg::PipelineId;
+use net_traits::ResourceThreads;
+use profile_traits::mem;
+use profile_traits::time;
+use script_traits::ScriptMsg;
+use script_traits::TimerSchedulerMsg;
+use servo_url::ImmutableOrigin;
+use servo_url::MutableOrigin;
+use servo_url::ServoUrl;
+
+#[dom_struct]
+/// https://drafts.css-houdini.org/worklets/#workletglobalscope
+pub struct WorkletGlobalScope {
+    /// The global for this worklet.
+    globalscope: GlobalScope,
+    /// The base URL for this worklet.
+    base_url: ServoUrl,
+    /// The microtask queue for this worklet
+    microtask_queue: MicrotaskQueue,
+}
+
+impl WorkletGlobalScope {
+    /// Create a new stack-allocated `WorkletGlobalScope`.
+    pub fn new_inherited(pipeline_id: PipelineId,
+                         base_url: ServoUrl,
+                         init: &WorkletGlobalScopeInit)
+                         -> WorkletGlobalScope {
+        // Any timer events fired on this global are ignored.
+        let (timer_event_chan, _) = ipc::channel().unwrap();
+        WorkletGlobalScope {
+            globalscope: GlobalScope::new_inherited(pipeline_id,
+                                                    init.devtools_chan.clone(),
+                                                    init.mem_profiler_chan.clone(),
+                                                    init.time_profiler_chan.clone(),
+                                                    init.constellation_chan.clone(),
+                                                    init.scheduler_chan.clone(),
+                                                    init.resource_threads.clone(),
+                                                    timer_event_chan,
+                                                    MutableOrigin::new(ImmutableOrigin::new_opaque())),
+            base_url: base_url,
+            microtask_queue: MicrotaskQueue::default(),
+        }
+    }
+
+    /// Evaluate a JS script in this global.
+    pub fn evaluate_js(&self, script: &str) -> bool {
+        debug!("Evaluating JS.");
+        rooted!(in (self.globalscope.get_cx()) let mut rval = UndefinedValue());
+        self.globalscope.evaluate_js_on_global_with_result(&*script, rval.handle_mut())
+    }
+
+    /// The base URL of this global.
+    pub fn base_url(&self) -> ServoUrl {
+        self.base_url.clone()
+    }
+
+    /// Queue up a microtask to be executed in this global.
+    pub fn enqueue_microtask(&self, job: Microtask) {
+        self.microtask_queue.enqueue(job);
+    }
+
+    /// Perform any queued microtasks.
+    pub fn perform_a_microtask_checkpoint(&self) {
+        self.microtask_queue.checkpoint(|id| {
+            let global = self.upcast::<GlobalScope>();
+            assert_eq!(global.pipeline_id(), id);
+            Some(Root::from_ref(global))
+        });
+    }
+
+    /// Perform a worklet task
+    pub fn perform_a_worklet_task(&self, task: WorkletTask) {
+        match task {
+            WorkletTask::Test(task) => match self.downcast::<TestWorkletGlobalScope>() {
+                Some(global) => global.perform_a_worklet_task(task),
+                None => warn!("This is not a test worklet."),
+            },
+        }
+    }
+}
+
+/// Resources required by workletglobalscopes
+#[derive(Clone)]
+pub struct WorkletGlobalScopeInit {
+    /// Channel to a resource thread
+    pub resource_threads: ResourceThreads,
+    /// Channel to the memory profiler
+    pub mem_profiler_chan: mem::ProfilerChan,
+    /// Channel to the time profiler
+    pub time_profiler_chan: time::ProfilerChan,
+    /// Channel to devtools
+    pub devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
+    /// Messages to send to constellation
+    pub constellation_chan: IpcSender<ScriptMsg>,
+    /// Message to send to the scheduler
+    pub scheduler_chan: IpcSender<TimerSchedulerMsg>,
+}
+
+/// https://drafts.css-houdini.org/worklets/#worklet-global-scope-type
+#[derive(Clone, Copy, Debug, HeapSizeOf, JSTraceable)]
+pub enum WorkletGlobalScopeType {
+    /// https://drafts.css-houdini.org/worklets/#examples
+    Test,
+}
+
+impl WorkletGlobalScopeType {
+    /// Create a new heap-allocated `WorkletGlobalScope`.
+    pub fn new(&self,
+               runtime: &Runtime,
+               pipeline_id: PipelineId,
+               base_url: ServoUrl,
+               init: &WorkletGlobalScopeInit)
+               -> Root<WorkletGlobalScope>
+    {
+        match *self {
+            WorkletGlobalScopeType::Test =>
+                Root::upcast(TestWorkletGlobalScope::new(runtime, pipeline_id, base_url, init)),
+        }
+    }
+}
+
+/// A task which can be performed in the context of a worklet global.
+pub enum WorkletTask {
+    Test(TestWorkletTask),
+}
+
diff --git a/components/script/lib.rs b/components/script/lib.rs
index c1bd886b927..703209604b0 100644
--- a/components/script/lib.rs
+++ b/components/script/lib.rs
@@ -10,10 +10,12 @@
 #![feature(nonzero)]
 #![feature(on_unimplemented)]
 #![feature(optin_builtin_traits)]
+#![feature(option_entry)]
 #![feature(plugin)]
 #![feature(proc_macro)]
 #![feature(stmt_expr_attributes)]
 #![feature(try_from)]
+#![feature(unboxed_closures)]
 #![feature(untagged_unions)]
 
 #![deny(unsafe_code)]
@@ -46,7 +48,7 @@ extern crate encoding;
 extern crate euclid;
 extern crate fnv;
 extern crate gfx_traits;
-extern crate heapsize;
+#[macro_use] extern crate heapsize;
 #[macro_use] extern crate heapsize_derive;
 #[macro_use] extern crate html5ever;
 #[macro_use]
@@ -92,6 +94,7 @@ extern crate smallvec;
 #[macro_use]
 extern crate style;
 extern crate style_traits;
+extern crate swapper;
 extern crate time;
 #[cfg(any(target_os = "macos", target_os = "linux", target_os = "windows"))]
 extern crate tinyfiledialogs;
diff --git a/components/script/script_runtime.rs b/components/script/script_runtime.rs
index c4f90edf8ea..8c243b18695 100644
--- a/components/script/script_runtime.rs
+++ b/components/script/script_runtime.rs
@@ -74,6 +74,7 @@ pub enum ScriptThreadEventCategory {
     UpdateReplacedElement,
     WebSocketEvent,
     WorkerEvent,
+    WorkletEvent,
     ServiceWorkerEvent,
     EnterFullscreen,
     ExitFullscreen,
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index a8b20b6cf98..8a6efa946ad 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -56,6 +56,8 @@ use dom::uievent::UIEvent;
 use dom::window::{ReflowReason, Window};
 use dom::windowproxy::WindowProxy;
 use dom::worker::TrustedWorkerAddress;
+use dom::worklet::WorkletThreadPool;
+use dom::workletglobalscope::WorkletGlobalScopeInit;
 use euclid::Rect;
 use euclid::point::Point2D;
 use hyper::header::{ContentType, HttpDate, LastModified, Headers};
@@ -490,6 +492,9 @@ pub struct ScriptThread {
     /// A handle to the webvr thread, if available
     webvr_thread: Option<IpcSender<WebVRMsg>>,
 
+    /// The worklet thread pool
+    worklet_thread_pool: DOMRefCell<Option<Rc<WorkletThreadPool>>>,
+
     /// A list of pipelines containing documents that finished loading all their blocking
     /// resources during a turn of the event loop.
     docs_with_no_blocking_loads: DOMRefCell<HashSet<JS<Document>>>,
@@ -703,6 +708,24 @@ impl ScriptThread {
         }))
     }
 
+    pub fn worklet_thread_pool() -> Rc<WorkletThreadPool> {
+        SCRIPT_THREAD_ROOT.with(|root| {
+            let script_thread = unsafe { &*root.get().unwrap() };
+            script_thread.worklet_thread_pool.borrow_mut().get_or_insert_with(|| {
+                let chan = script_thread.chan.0.clone();
+                let init = WorkletGlobalScopeInit {
+                    resource_threads: script_thread.resource_threads.clone(),
+                    mem_profiler_chan: script_thread.mem_profiler_chan.clone(),
+                    time_profiler_chan: script_thread.time_profiler_chan.clone(),
+                    devtools_chan: script_thread.devtools_chan.clone(),
+                    constellation_chan: script_thread.constellation_chan.clone(),
+                    scheduler_chan: script_thread.scheduler_chan.clone(),
+                };
+                Rc::new(WorkletThreadPool::spawn(chan, init))
+            }).clone()
+        })
+    }
+
     /// Creates a new script thread.
     pub fn new(state: InitialScriptState,
                port: Receiver<MainThreadScriptMsg>,
@@ -782,6 +805,8 @@ impl ScriptThread {
 
             webvr_thread: state.webvr_thread,
 
+            worklet_thread_pool: Default::default(),
+
             docs_with_no_blocking_loads: Default::default(),
 
             transitioning_nodes: Default::default(),
@@ -1065,6 +1090,7 @@ impl ScriptThread {
                 ScriptThreadEventCategory::WebSocketEvent => ProfilerCategory::ScriptWebSocketEvent,
                 ScriptThreadEventCategory::WebVREvent => ProfilerCategory::ScriptWebVREvent,
                 ScriptThreadEventCategory::WorkerEvent => ProfilerCategory::ScriptWorkerEvent,
+                ScriptThreadEventCategory::WorkletEvent => ProfilerCategory::ScriptWorkletEvent,
                 ScriptThreadEventCategory::ServiceWorkerEvent => ProfilerCategory::ScriptServiceWorkerEvent,
                 ScriptThreadEventCategory::EnterFullscreen => ProfilerCategory::ScriptEnterFullscreen,
                 ScriptThreadEventCategory::ExitFullscreen => ProfilerCategory::ScriptExitFullscreen,
@@ -1149,7 +1175,7 @@ impl ScriptThread {
                 // The category of the runnable is ignored by the pattern, however
                 // it is still respected by profiling (see categorize_msg).
                 if !runnable.is_cancelled() {
-                    runnable.handler()
+                    runnable.main_thread_handler(self)
                 }
             }
             MainThreadScriptMsg::Common(CommonScriptMsg::CollectReports(reports_chan)) =>
diff --git a/components/script_traits/Cargo.toml b/components/script_traits/Cargo.toml
index da5fcc78704..993c14c43b0 100644
--- a/components/script_traits/Cargo.toml
+++ b/components/script_traits/Cargo.toml
@@ -10,7 +10,6 @@ name = "script_traits"
 path = "lib.rs"
 
 [dependencies]
-app_units = "0.4"
 bluetooth_traits = {path = "../bluetooth_traits"}
 canvas_traits = {path = "../canvas_traits"}
 cookie = "0.6"
diff --git a/python/tidy/servo_tidy/tidy.py b/python/tidy/servo_tidy/tidy.py
index ccca233db9c..b8229e67d96 100644
--- a/python/tidy/servo_tidy/tidy.py
+++ b/python/tidy/servo_tidy/tidy.py
@@ -67,6 +67,7 @@ WEBIDL_STANDARDS = [
     "//dom.spec.whatwg.org",
     "//domparsing.spec.whatwg.org",
     "//drafts.csswg.org",
+    "//drafts.css-houdini.org",
     "//drafts.fxtf.org",
     "//encoding.spec.whatwg.org",
     "//fetch.spec.whatwg.org",
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index ce611aee8f1..c94175fdce8 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -11185,6 +11185,21 @@
     [
      {}
     ]
+   ],
+   "mozilla/worklets/syntax_error.js": [
+    [
+     {}
+    ]
+   ],
+   "mozilla/worklets/test_worklet.js": [
+    [
+     {}
+    ]
+   ],
+   "mozilla/worklets/throw_exception.js": [
+    [
+     {}
+    ]
    ]
   },
   "testharness": {
@@ -20027,6 +20042,12 @@
      "/_mozilla/mozilla/windowproxy.html",
      {}
     ]
+   ],
+   "mozilla/worklets/test_worklet.html": [
+    [
+     "/_mozilla/mozilla/worklets/test_worklet.html",
+     {}
+    ]
    ]
   }
  },
@@ -25796,7 +25817,7 @@
    "testharness"
   ],
   "mozilla/interfaces.html": [
-   "21e18bafdbfe5f3aa0ee71766bdc3b6a7e334226",
+   "49dd9f6ef449813f2ce943d4c9fac351357e5c74",
    "testharness"
   ],
   "mozilla/interfaces.js": [
@@ -31714,6 +31735,22 @@
   "mozilla/windowproxy.html": [
    "128cd0aa5cf80f60078979039036d32b470b0616",
    "testharness"
+  ],
+  "mozilla/worklets/syntax_error.js": [
+   "f3a9b8c78346507bc0b3190c8000ccf80cc133f6",
+   "support"
+  ],
+  "mozilla/worklets/test_worklet.html": [
+   "fe9c93a5307c616f878b6623155e1b04c86dd994",
+   "testharness"
+  ],
+  "mozilla/worklets/test_worklet.js": [
+   "9d5f8a07cd62a10f4f5ff93744672e5a6fdbc2b0",
+   "support"
+  ],
+  "mozilla/worklets/throw_exception.js": [
+   "ebfdae19db68fed8e69142ef73842ac9921e4463",
+   "support"
   ]
  },
  "url_base": "/_mozilla/",
diff --git a/tests/wpt/mozilla/meta/mozilla/worklets/test_worklet.html.ini b/tests/wpt/mozilla/meta/mozilla/worklets/test_worklet.html.ini
new file mode 100644
index 00000000000..37762a513dd
--- /dev/null
+++ b/tests/wpt/mozilla/meta/mozilla/worklets/test_worklet.html.ini
@@ -0,0 +1,3 @@
+[test_worklet.html]
+  type: testharness
+  prefs: [dom.worklet.testing.enabled:true]
diff --git a/tests/wpt/mozilla/tests/mozilla/interfaces.html b/tests/wpt/mozilla/tests/mozilla/interfaces.html
index a29f439a7c6..ee987ae9322 100644
--- a/tests/wpt/mozilla/tests/mozilla/interfaces.html
+++ b/tests/wpt/mozilla/tests/mozilla/interfaces.html
@@ -200,6 +200,7 @@ test_interfaces([
   "WebSocket",
   "Window",
   "Worker",
+  "Worklet",
   "XMLDocument",
   "XMLHttpRequest",
   "XMLHttpRequestEventTarget",
diff --git a/tests/wpt/mozilla/tests/mozilla/worklets/syntax_error.js b/tests/wpt/mozilla/tests/mozilla/worklets/syntax_error.js
new file mode 100644
index 00000000000..4adade8939c
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/worklets/syntax_error.js
@@ -0,0 +1 @@
+{];
diff --git a/tests/wpt/mozilla/tests/mozilla/worklets/test_worklet.html b/tests/wpt/mozilla/tests/mozilla/worklets/test_worklet.html
new file mode 100644
index 00000000000..d7a9efa04fe
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/worklets/test_worklet.html
@@ -0,0 +1,35 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Test worklet loading</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/get-host-info.sub.js"></script>
+<script>
+var testWorklet = new TestWorklet();
+var host_info = get_host_info();
+
+promise_test(function() {
+  return testWorklet.addModule("test_worklet.js")
+    .then(function () {
+      assert_equals(testWorklet.lookup("hello"), "world");
+    });
+}, "Loading a test worklet.");
+
+promise_test(function(t) {
+   var path = new URL("test_worklet.js", document.location).pathname;
+   var url = new URL(path, host_info.HTTP_REMOTE_ORIGIN);
+   return promise_rejects(t, "AbortError", testWorklet.addModule(url));
+}, "Loading a cross-origin test worklet.");
+
+promise_test(function(t) {
+   return promise_rejects(t, "AbortError", testWorklet.addModule("nonexistent_worklet.js"));
+}, "Loading a nonexistent test worklet.");
+
+promise_test(function(t) {
+   return promise_rejects(t, "AbortError", testWorklet.addModule("syntax_error.js"));
+}, "Loading a syntactically incorrect test worklet.");
+
+promise_test(function(t) {
+   return promise_rejects(t, "AbortError", testWorklet.addModule("throw_exception.js"));
+}, "Loading an exception-throwing test worklet.");
+</script>
diff --git a/tests/wpt/mozilla/tests/mozilla/worklets/test_worklet.js b/tests/wpt/mozilla/tests/mozilla/worklets/test_worklet.js
new file mode 100644
index 00000000000..9c0b392a6ab
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/worklets/test_worklet.js
@@ -0,0 +1 @@
+registerKeyValue("hello", "world");
diff --git a/tests/wpt/mozilla/tests/mozilla/worklets/throw_exception.js b/tests/wpt/mozilla/tests/mozilla/worklets/throw_exception.js
new file mode 100644
index 00000000000..6ca4f80fc27
--- /dev/null
+++ b/tests/wpt/mozilla/tests/mozilla/worklets/throw_exception.js
@@ -0,0 +1 @@
+throw new TypeError();