Auto merge of #12392 - emilio:test-animations, r=SimonSapin

style: Add support to test animations programatically.

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---
<!-- 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 #12120

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- [x] There are tests for these changes OR

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r? @SimonSapin for the style changes, @Ms2ger or @jdm for the dom and test changes

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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12392)
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22 files changed, 216 insertions, 36 deletions

diff --git a/components/layout/Cargo.toml b/components/layout/Cargo.toml
index 3add6b14976..9c83648128a 100644
--- a/components/layout/Cargo.toml
+++ b/components/layout/Cargo.toml
@@ -38,7 +38,6 @@ smallvec = "0.1"
 string_cache = {version = "0.2.20", features = ["heap_size"]}
 style = {path = "../style"}
 style_traits = {path = "../style_traits"}
-time = "0.1"
 unicode-bidi = "0.2"
 unicode-script = {version = "0.1", features = ["harfbuzz"]}
 url = {version = "1.0.0", features = ["heap_size"]}
diff --git a/components/layout/animation.rs b/components/layout/animation.rs
index 4087479ed47..5a67b1af5bf 100644
--- a/components/layout/animation.rs
+++ b/components/layout/animation.rs
@@ -14,7 +14,7 @@ use script_traits::{AnimationState, LayoutMsg as ConstellationMsg};
 use std::collections::HashMap;
 use std::sync::mpsc::Receiver;
 use style::animation::{Animation, update_style_for_animation};
-use time;
+use style::timer::Timer;
 
 /// Processes any new animations that were discovered after style recalculation.
 /// Also expire any old animations that have completed, inserting them into
@@ -23,7 +23,8 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
                               running_animations: &mut HashMap<OpaqueNode, Vec<Animation>>,
                               expired_animations: &mut HashMap<OpaqueNode, Vec<Animation>>,
                               new_animations_receiver: &Receiver<Animation>,
-                              pipeline_id: PipelineId) {
+                              pipeline_id: PipelineId,
+                              timer: &Timer) {
     let mut new_running_animations = vec![];
     while let Ok(animation) = new_animations_receiver.try_recv() {
         let mut should_push = true;
@@ -37,7 +38,7 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
                     if let Animation::Keyframes(_, ref anim_name, ref mut anim_state) = *anim {
                         if *name == *anim_name {
                             debug!("update_animation_state: Found other animation {}", name);
-                            anim_state.update_from_other(&state);
+                            anim_state.update_from_other(&state, timer);
                             should_push = false;
                             break;
                         }
@@ -57,7 +58,7 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
         return
     }
 
-    let now = time::precise_time_s();
+    let now = timer.seconds();
     // Expire old running animations.
     //
     // TODO: Do not expunge Keyframes animations, since we need that state if
diff --git a/components/layout/lib.rs b/components/layout/lib.rs
index f05b21c3628..b6f10c7fa89 100644
--- a/components/layout/lib.rs
+++ b/components/layout/lib.rs
@@ -50,7 +50,6 @@ extern crate smallvec;
 #[macro_use(atom, ns)] extern crate string_cache;
 extern crate style;
 extern crate style_traits;
-extern crate time;
 extern crate unicode_bidi;
 extern crate unicode_script;
 extern crate url;
diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index 7fc12535ff7..592a6cda6a4 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -110,11 +110,13 @@ use style::refcell::RefCell;
 use style::selector_matching::Stylist;
 use style::servo_selector_impl::USER_OR_USER_AGENT_STYLESHEETS;
 use style::stylesheets::{Stylesheet, CSSRuleIteratorExt};
+use style::timer::Timer;
 use style::workqueue::WorkQueue;
 use url::Url;
 use util::geometry::MAX_RECT;
 use util::ipc::OptionalIpcSender;
 use util::opts;
+use util::prefs::PREFS;
 use util::thread;
 use util::thread_state;
 
@@ -226,6 +228,10 @@ pub struct LayoutThread {
 
     // Webrender interface, if enabled.
     webrender_api: Option<webrender_traits::RenderApi>,
+
+    /// The timer object to control the timing of the animations. This should
+    /// only be a test-mode timer during testing for animations.
+    timer: Timer,
 }
 
 impl LayoutThreadFactory for LayoutThread {
@@ -459,13 +465,20 @@ impl LayoutThread {
                     offset_parent_response: OffsetParentResponse::empty(),
                     margin_style_response: MarginStyleResponse::empty(),
                     stacking_context_scroll_offsets: HashMap::new(),
-              })),
-              error_reporter: CSSErrorReporter {
-                  pipelineid: id,
-                  script_chan: Arc::new(Mutex::new(script_chan)),
-              },
-              webrender_image_cache:
-                  Arc::new(RwLock::new(HashMap::with_hasher(Default::default()))),
+                })),
+            error_reporter: CSSErrorReporter {
+                pipelineid: id,
+                script_chan: Arc::new(Mutex::new(script_chan)),
+            },
+            webrender_image_cache:
+                Arc::new(RwLock::new(HashMap::with_hasher(Default::default()))),
+            timer:
+                if PREFS.get("layout.animations.test.enabled")
+                           .as_boolean().unwrap_or(false) {
+                   Timer::test_mode()
+                } else {
+                    Timer::new()
+                },
         }
     }
 
@@ -501,6 +514,7 @@ impl LayoutThread {
                 expired_animations: self.expired_animations.clone(),
                 error_reporter: self.error_reporter.clone(),
                 local_context_creation_data: Mutex::new(local_style_context_creation_data),
+                timer: self.timer.clone(),
             },
             image_cache_thread: self.image_cache_thread.clone(),
             image_cache_sender: Mutex::new(self.image_cache_sender.clone()),
@@ -653,6 +667,9 @@ impl LayoutThread {
                 let _rw_data = possibly_locked_rw_data.lock();
                 sender.send(self.epoch).unwrap();
             },
+            Msg::AdvanceClockMs(how_many) => {
+                self.handle_advance_clock_ms(how_many, possibly_locked_rw_data);
+            }
             Msg::GetWebFontLoadState(sender) => {
                 let _rw_data = possibly_locked_rw_data.lock();
                 let outstanding_web_fonts = self.outstanding_web_fonts.load(Ordering::SeqCst);
@@ -795,6 +812,14 @@ impl LayoutThread {
         possibly_locked_rw_data.block(rw_data);
     }
 
+    /// Advances the animation clock of the document.
+    fn handle_advance_clock_ms<'a, 'b>(&mut self,
+                                       how_many_ms: i32,
+                                       possibly_locked_rw_data: &mut RwData<'a, 'b>) {
+        self.timer.increment(how_many_ms as f64 / 1000.0);
+        self.tick_all_animations(possibly_locked_rw_data);
+    }
+
     /// Sets quirks mode for the document, causing the quirks mode stylesheet to be used.
     fn handle_set_quirks_mode<'a, 'b>(&self, possibly_locked_rw_data: &mut RwData<'a, 'b>) {
         let mut rw_data = possibly_locked_rw_data.lock();
@@ -1350,7 +1375,8 @@ impl LayoutThread {
                                               &mut *self.running_animations.write().unwrap(),
                                               &mut *self.expired_animations.write().unwrap(),
                                               &self.new_animations_receiver,
-                                              self.id);
+                                              self.id,
+                                              &self.timer);
 
             profile(time::ProfilerCategory::LayoutRestyleDamagePropagation,
                     self.profiler_metadata(),
diff --git a/components/script/dom/testbinding.rs b/components/script/dom/testbinding.rs
index 64fb6fb7578..8ee6dfe8025 100644
--- a/components/script/dom/testbinding.rs
+++ b/components/script/dom/testbinding.rs
@@ -581,6 +581,10 @@ impl TestBindingMethods for TestBinding {
         }
     }
 
+    fn AdvanceClock(&self, ms: i32) {
+        self.global().r().as_window().advance_animation_clock(ms);
+    }
+
     fn Panic(&self) { panic!("explicit panic from script") }
 }
 
diff --git a/components/script/dom/webidls/TestBinding.webidl b/components/script/dom/webidls/TestBinding.webidl
index 355141a4d92..48e647ee7bc 100644
--- a/components/script/dom/webidls/TestBinding.webidl
+++ b/components/script/dom/webidls/TestBinding.webidl
@@ -424,6 +424,8 @@ interface TestBinding {
   static void prefControlledStaticMethodDisabled();
   [Pref="dom.testbinding.prefcontrolled.enabled"]
   const unsigned short prefControlledConstDisabled = 0;
+  [Pref="layout.animations.test.enabled"]
+  void advanceClock(long millis);
 
   [Pref="dom.testbinding.prefcontrolled2.enabled"]
   readonly attribute boolean prefControlledAttributeEnabled;
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 4878f33f994..07509a92e4a 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -1030,6 +1030,12 @@ impl Window {
         recv.recv().unwrap_or((Size2D::zero(), Point2D::zero()))
     }
 
+    /// Advances the layout animation clock by `delta` milliseconds, and then
+    /// forces a reflow.
+    pub fn advance_animation_clock(&self, delta: i32) {
+        self.layout_chan.send(Msg::AdvanceClockMs(delta)).unwrap();
+    }
+
     /// Reflows the page unconditionally if possible and not suppressed. This
     /// method will wait for the layout thread to complete (but see the `TODO`
     /// below). If there is no window size yet, the page is presumed invisible
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index ec2401d6f61..1a32477b999 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -886,22 +886,12 @@ impl ScriptThread {
 
     fn handle_msg_from_constellation(&self, msg: ConstellationControlMsg) {
         match msg {
-            ConstellationControlMsg::AttachLayout(_) =>
-                panic!("should have handled AttachLayout already"),
             ConstellationControlMsg::Navigate(pipeline_id, subpage_id, load_data) =>
                 self.handle_navigate(pipeline_id, Some(subpage_id), load_data),
             ConstellationControlMsg::SendEvent(id, event) =>
                 self.handle_event(id, event),
             ConstellationControlMsg::ResizeInactive(id, new_size) =>
                 self.handle_resize_inactive_msg(id, new_size),
-            ConstellationControlMsg::Viewport(..) =>
-                panic!("should have handled Viewport already"),
-            ConstellationControlMsg::SetScrollState(..) =>
-                panic!("should have handled SetScrollState already"),
-            ConstellationControlMsg::Resize(..) =>
-                panic!("should have handled Resize already"),
-            ConstellationControlMsg::ExitPipeline(..) =>
-                panic!("should have handled ExitPipeline already"),
             ConstellationControlMsg::GetTitle(pipeline_id) =>
                 self.handle_get_title_msg(pipeline_id),
             ConstellationControlMsg::Freeze(pipeline_id) =>
@@ -943,6 +933,12 @@ impl ScriptThread {
                 self.handle_css_error_reporting(pipeline_id, filename, line, column, msg),
             ConstellationControlMsg::Reload(pipeline_id) =>
                 self.handle_reload(pipeline_id),
+            msg @ ConstellationControlMsg::AttachLayout(..) |
+            msg @ ConstellationControlMsg::Viewport(..) |
+            msg @ ConstellationControlMsg::SetScrollState(..) |
+            msg @ ConstellationControlMsg::Resize(..) |
+            msg @ ConstellationControlMsg::ExitPipeline(..) =>
+                      panic!("should have handled {:?} already", msg),
         }
     }
 
diff --git a/components/script_layout_interface/message.rs b/components/script_layout_interface/message.rs
index 22370b043c3..ce97c805bb2 100644
--- a/components/script_layout_interface/message.rs
+++ b/components/script_layout_interface/message.rs
@@ -40,6 +40,11 @@ pub enum Msg {
     /// Requests that the layout thread render the next frame of all animations.
     TickAnimations,
 
+    /// Updates layout's timer for animation testing from script.
+    ///
+    /// The inner field is the number of *milliseconds* to advance.
+    AdvanceClockMs(i32),
+
     /// Requests that the layout thread reflow with a newly-loaded Web font.
     ReflowWithNewlyLoadedWebFont,
 
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index f57ed29f43c..175cc0b82f2 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -61,6 +61,7 @@ use profile_traits::mem;
 use profile_traits::time as profile_time;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use std::collections::HashMap;
+use std::fmt;
 use std::sync::mpsc::{Sender, Receiver};
 use style_traits::{PagePx, ViewportPx};
 use url::Url;
@@ -207,6 +208,37 @@ pub enum ConstellationControlMsg {
     Reload(PipelineId),
 }
 
+impl fmt::Debug for ConstellationControlMsg {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        use self::ConstellationControlMsg::*;
+        write!(formatter, "ConstellationMsg::{}", match *self {
+            AttachLayout(..) => "AttachLayout",
+            Resize(..) => "Resize",
+            ResizeInactive(..) => "ResizeInactive",
+            ExitPipeline(..) => "ExitPipeline",
+            SendEvent(..) => "SendEvent",
+            Viewport(..) => "Viewport",
+            SetScrollState(..) => "SetScrollState",
+            GetTitle(..) => "GetTitle",
+            Freeze(..) => "Freeze",
+            Thaw(..) => "Thaw",
+            ChangeFrameVisibilityStatus(..) => "ChangeFrameVisibilityStatus",
+            NotifyVisibilityChange(..) => "NotifyVisibilityChange",
+            Navigate(..) => "Navigate",
+            MozBrowserEvent(..) => "MozBrowserEvent",
+            UpdateSubpageId(..) => "UpdateSubpageId",
+            FocusIFrame(..) => "FocusIFrame",
+            WebDriverScriptCommand(..) => "WebDriverScriptCommand",
+            TickAllAnimations(..) => "TickAllAnimations",
+            WebFontLoaded(..) => "WebFontLoaded",
+            DispatchFrameLoadEvent { .. } => "DispatchFrameLoadEvent",
+            FramedContentChanged(..) => "FramedContentChanged",
+            ReportCSSError(..) => "ReportCSSError",
+            Reload(..) => "Reload",
+        })
+    }
+}
+
 /// Used to determine if a script has any pending asynchronous activity.
 #[derive(Copy, Clone, Debug, PartialEq, Deserialize, Serialize)]
 pub enum DocumentState {
diff --git a/components/servo/Cargo.lock b/components/servo/Cargo.lock
index 55a1d808087..d7eb68f237a 100644
--- a/components/servo/Cargo.lock
+++ b/components/servo/Cargo.lock
@@ -1136,7 +1136,6 @@ dependencies = [
  "string_cache 0.2.21 (registry+https://github.com/rust-lang/crates.io-index)",
  "style 0.0.1",
  "style_traits 0.0.1",
- "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicode-bidi 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicode-script 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/components/style/animation.rs b/components/style/animation.rs
index c9529f7b99f..c36cde3caea 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -20,7 +20,7 @@ use selectors::matching::DeclarationBlock;
 use std::sync::Arc;
 use std::sync::mpsc::Sender;
 use string_cache::Atom;
-use time;
+use timer::Timer;
 use values::computed::Time;
 
 /// This structure represents a keyframes animation current iteration state.
@@ -122,7 +122,9 @@ impl KeyframesAnimationState {
     ///
     /// There are some bits of state we can't just replace, over all taking in
     /// account times, so here's that logic.
-    pub fn update_from_other(&mut self, other: &Self) {
+    pub fn update_from_other(&mut self,
+                             other: &Self,
+                             timer: &Timer) {
         use self::KeyframesRunningState::*;
 
         debug!("KeyframesAnimationState::update_from_other({:?}, {:?})", self, other);
@@ -146,11 +148,11 @@ impl KeyframesAnimationState {
         // If we're pausing the animation, compute the progress value.
         match (&mut self.running_state, old_running_state) {
             (&mut Running, Paused(progress))
-                => new_started_at = time::precise_time_s() - (self.duration * progress),
+                => new_started_at = timer.seconds() - (self.duration * progress),
             (&mut Paused(ref mut new), Paused(old))
                 => *new = old,
             (&mut Paused(ref mut progress), Running)
-                => *progress = (time::precise_time_s() - old_started_at) / old_duration,
+                => *progress = (timer.seconds() - old_started_at) / old_duration,
             _ => {},
         }
 
@@ -341,7 +343,8 @@ impl PropertyAnimation {
 pub fn start_transitions_if_applicable(new_animations_sender: &Sender<Animation>,
                                        node: OpaqueNode,
                                        old_style: &ComputedValues,
-                                       new_style: &mut Arc<ComputedValues>)
+                                       new_style: &mut Arc<ComputedValues>,
+                                       timer: &Timer)
                                        -> bool {
     let mut had_animations = false;
     for i in 0..new_style.get_box().transition_property_count() {
@@ -355,7 +358,7 @@ pub fn start_transitions_if_applicable(new_animations_sender: &Sender<Animation>
 
             // Kick off the animation.
             let box_style = new_style.get_box();
-            let now = time::precise_time_s();
+            let now = timer.seconds();
             let start_time =
                 now + (box_style.transition_delay_mod(i).seconds() as f64);
             new_animations_sender
@@ -424,7 +427,7 @@ pub fn maybe_start_animations(context: &SharedStyleContext,
             }
 
             let delay = box_style.animation_delay_mod(i).seconds();
-            let now = time::precise_time_s();
+            let now = context.timer.seconds();
             let animation_start = now + delay as f64;
             let duration = box_style.animation_duration_mod(i).seconds();
             let iteration_state = match box_style.animation_iteration_count_mod(i) {
@@ -497,7 +500,7 @@ where Damage: TRestyleDamage {
     match *animation {
         Animation::Transition(_, start_time, ref frame, _) => {
             debug!("update_style_for_animation: transition found");
-            let now = time::precise_time_s();
+            let now = context.timer.seconds();
             let mut new_style = (*style).clone();
             let updated_style = update_style_for_animation_frame(&mut new_style,
                                                                  now, start_time,
@@ -516,7 +519,7 @@ where Damage: TRestyleDamage {
             let started_at = state.started_at;
 
             let now = match state.running_state {
-                KeyframesRunningState::Running => time::precise_time_s(),
+                KeyframesRunningState::Running => context.timer.seconds(),
                 KeyframesRunningState::Paused(progress) => started_at + duration * progress,
             };
 
diff --git a/components/style/context.rs b/components/style/context.rs
index 3515728b4cf..f1c5ac3f307 100644
--- a/components/style/context.rs
+++ b/components/style/context.rs
@@ -15,6 +15,7 @@ use std::cell::RefCell;
 use std::collections::HashMap;
 use std::sync::mpsc::Sender;
 use std::sync::{Arc, Mutex, RwLock};
+use timer::Timer;
 
 /// This structure is used to create a local style context from a shared one.
 pub struct LocalStyleContextCreationInfo {
@@ -57,6 +58,10 @@ pub struct SharedStyleContext {
 
     /// Data needed to create the local style context from the shared one.
     pub local_context_creation_data: Mutex<LocalStyleContextCreationInfo>,
+
+    /// The current timer for transitions and animations. This is needed to test
+    /// them.
+    pub timer: Timer,
 }
 
 pub struct LocalStyleContext {
diff --git a/components/style/lib.rs b/components/style/lib.rs
index 673e8311b02..1ad47061062 100644
--- a/components/style/lib.rs
+++ b/components/style/lib.rs
@@ -101,6 +101,7 @@ pub mod sink;
 pub mod str;
 pub mod stylesheets;
 mod tid;
+pub mod timer;
 pub mod traversal;
 #[macro_use]
 #[allow(non_camel_case_types)]
diff --git a/components/style/matching.rs b/components/style/matching.rs
index 42320d2ecdd..2c4dfc990cd 100644
--- a/components/style/matching.rs
+++ b/components/style/matching.rs
@@ -435,7 +435,8 @@ trait PrivateMatchMethods: TNode
                         new_animations_sender,
                         this_opaque,
                         &**style,
-                        &mut this_style);
+                        &mut this_style,
+                        &shared_context.timer);
             }
 
             cacheable = cacheable && !animations_started
diff --git a/components/style/timer.rs b/components/style/timer.rs
new file mode 100644
index 00000000000..619e93e80f1
--- /dev/null
+++ b/components/style/timer.rs
@@ -0,0 +1,58 @@
+/* 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 time;
+
+/// The `TimerMode` is used to determine what time should the `Timer` return,
+/// either a fixed value (in the `Test` mode), or the actual time (in the
+/// `Current` mode).
+#[derive(Debug, Clone)]
+enum TimerMode {
+    Test(f64),
+    Current,
+}
+
+/// A `Timer` struct that takes care of giving the current time for animations.
+///
+/// This is needed to be allowed to hook the time in the animations' test-mode.
+#[derive(Debug, Clone)]
+pub struct Timer {
+    mode: TimerMode,
+}
+
+impl Timer {
+    /// Creates a new "normal" timer, i.e., a "Current" mode timer.
+    #[inline]
+    pub fn new() -> Self {
+        Timer {
+            mode: TimerMode::Current,
+        }
+    }
+
+    /// Creates a new "test mode" timer, with initial time 0.
+    #[inline]
+    pub fn test_mode() -> Self {
+        Timer {
+            mode: TimerMode::Test(0.),
+        }
+    }
+
+    /// Returns the current time, at least from the caller's perspective. In
+    /// test mode returns whatever the value is.
+    pub fn seconds(&self) -> f64 {
+        match self.mode {
+            TimerMode::Test(test_value) => test_value,
+            TimerMode::Current => time::precise_time_s(),
+        }
+    }
+
+    /// Increments the current clock. Panics if the clock is not on test mode.
+    pub fn increment(&mut self, by: f64) {
+        match self.mode {
+            TimerMode::Test(ref mut val)
+                => *val += by,
+            TimerMode::Current
+                => panic!("Timer::increment called for a non-test mode timer. This is a bug."),
+        }
+    }
+}
diff --git a/ports/cef/Cargo.lock b/ports/cef/Cargo.lock
index cc6112d0ef8..591371cbfc0 100644
--- a/ports/cef/Cargo.lock
+++ b/ports/cef/Cargo.lock
@@ -1045,7 +1045,6 @@ dependencies = [
  "string_cache 0.2.21 (registry+https://github.com/rust-lang/crates.io-index)",
  "style 0.0.1",
  "style_traits 0.0.1",
- "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicode-bidi 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "unicode-script 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index 9d8e8ec5129..f1da1fe14b5 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -31,6 +31,7 @@ use style::properties::{ComputedValues, PropertyDeclarationBlock, parse_one_decl
 use style::selector_impl::{SelectorImplExt, PseudoElementCascadeType};
 use style::sequential;
 use style::stylesheets::{Stylesheet, Origin};
+use style::timer::Timer;
 use traversal::RecalcStyleOnly;
 use url::Url;
 use wrapper::{DUMMY_BASE_URL, GeckoDocument, GeckoElement, GeckoNode, NonOpaqueStyleData};
@@ -106,6 +107,7 @@ fn restyle_subtree(node: GeckoNode, raw_data: *mut RawServoStyleSet) {
         expired_animations: per_doc_data.expired_animations.clone(),
         error_reporter: Box::new(StdoutErrorReporter),
         local_context_creation_data: Mutex::new(local_context_data),
+        timer: Timer::new(),
     };
 
     if node.is_dirty() || node.has_dirty_descendants() {
diff --git a/resources/prefs.json b/resources/prefs.json
index 64ac5353cdb..35961dee0e2 100644
--- a/resources/prefs.json
+++ b/resources/prefs.json
@@ -45,6 +45,7 @@
   "js.mem.gc.empty_chunk_count_max": 30,
   "js.mem.gc.zeal.level": 0,
   "js.mem.gc.zeal.frequency": 100,
+  "layout.animations.test.enabled": false,
   "layout.columns.enabled": false,
   "layout.column-width.enabled": false,
   "layout.column-count.enabled": false,
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index d11b5f3e1cc..3fedff120e5 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -6120,6 +6120,12 @@
         ]
       },
       "testharness": {
+        "css/animations/basic-linear-width.html": [
+          {
+            "path": "css/animations/basic-linear-width.html",
+            "url": "/_mozilla/css/animations/basic-linear-width.html"
+          }
+        ],
         "css/empty-keyframes.html": [
           {
             "path": "css/empty-keyframes.html",
diff --git a/tests/wpt/mozilla/meta/css/animations/__dir__.ini b/tests/wpt/mozilla/meta/css/animations/__dir__.ini
new file mode 100644
index 00000000000..696581b4a4a
--- /dev/null
+++ b/tests/wpt/mozilla/meta/css/animations/__dir__.ini
@@ -0,0 +1,2 @@
+prefs: ["layout.animations.test.enabled:true",
+        "dom.testbinding.enabled:true"]
diff --git a/tests/wpt/mozilla/tests/css/animations/basic-linear-width.html b/tests/wpt/mozilla/tests/css/animations/basic-linear-width.html
new file mode 100644
index 00000000000..634b09dca59
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/animations/basic-linear-width.html
@@ -0,0 +1,33 @@
+<!doctype html>
+<meta charset="utf-8">
+<title>Animation test: Linear animation repeated infinitely.</title>
+<style>
+  .animatable {
+    width: 50px;
+    height: 50px;
+    background: red;
+    animation: foo 1s infinite linear;
+  }
+  @keyframes foo {
+    from { width: 0; }
+    to { width: 500px; }
+  }
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div class="animatable"></div>
+<script>
+var div = document.querySelector('.animatable');
+async_test(function(t) {
+  window.addEventListener('load', function() {
+    var test = new window.TestBinding();
+    test.advanceClock(500);
+    assert_equals(getComputedStyle(div).getPropertyValue('width'), '250px');
+    test.advanceClock(500);
+    assert_equals(getComputedStyle(div).getPropertyValue('width'), '500px');
+    test.advanceClock(500);
+    assert_equals(getComputedStyle(div).getPropertyValue('width'), '250px');
+    t.done();
+  })
+})
+</script>