Add support for animationend event

This is triggered when an animation finishes. This is a high priority
because it allows us to start rooting nodes with animations in the
script thread.

This doesn't yet cause a lot of tests to pass because they rely on the
existence of `Document.getAnimations()` and the presence of
`animationstart` and animationiteration` events.

27 files changed, 334 insertions, 330 deletions

diff --git a/components/layout/animation.rs b/components/layout/animation.rs
index 00fbf4bc1d6..64c1dfd2e0a 100644
--- a/components/layout/animation.rs
+++ b/components/layout/animation.rs
@@ -13,20 +13,20 @@ use ipc_channel::ipc::IpcSender;
 use msg::constellation_msg::PipelineId;
 use script_traits::UntrustedNodeAddress;
 use script_traits::{
-    AnimationState, ConstellationControlMsg, LayoutMsg as ConstellationMsg, TransitionEventType,
+    AnimationState, ConstellationControlMsg, LayoutMsg as ConstellationMsg,
+    TransitionOrAnimationEventType,
 };
 use servo_arc::Arc;
-use style::animation::{
-    update_style_for_animation, Animation, ElementAnimationState, PropertyAnimation,
-};
+use style::animation::{update_style_for_animation, Animation, ElementAnimationState};
 use style::dom::TElement;
 use style::font_metrics::ServoMetricsProvider;
 use style::selector_parser::RestyleDamage;
 use style::timer::Timer;
 
-/// Collect newly transitioning nodes, which is used by the script process during
-/// forced, synchronous reflows to root DOM nodes for the duration of their transitions.
-pub fn collect_newly_transitioning_nodes(
+/// Collect newly animating nodes, which is used by the script process during
+/// forced, synchronous reflows to root DOM nodes for the duration of their
+/// animations or transitions.
+pub fn collect_newly_animating_nodes(
     animation_states: &FxHashMap<OpaqueNode, ElementAnimationState>,
     mut out: Option<&mut Vec<UntrustedNodeAddress>>,
 ) {
@@ -35,12 +35,7 @@ pub fn collect_newly_transitioning_nodes(
     // currently stores a rooted node for every property that is transitioning.
     if let Some(ref mut out) = out {
         out.extend(animation_states.iter().flat_map(|(node, state)| {
-            let num_transitions = state
-                .new_animations
-                .iter()
-                .filter(|animation| animation.is_transition())
-                .count();
-            std::iter::repeat(node.to_untrusted_node_address()).take(num_transitions)
+            std::iter::repeat(node.to_untrusted_node_address()).take(state.new_animations.len())
         }));
     }
 }
@@ -101,21 +96,28 @@ pub fn update_animation_state(
     now: f64,
     node: OpaqueNode,
 ) {
-    let send_transition_event = |property_animation: &PropertyAnimation, event_type| {
+    let send_event = |animation: &Animation, event_type, elapsed_time| {
+        let property_or_animation_name = match *animation {
+            Animation::Transition(_, _, ref property_animation) => {
+                property_animation.property_name().into()
+            },
+            Animation::Keyframes(_, _, ref name, _) => name.to_string(),
+        };
+
         script_channel
-            .send(ConstellationControlMsg::TransitionEvent {
+            .send(ConstellationControlMsg::TransitionOrAnimationEvent {
                 pipeline_id,
                 event_type,
                 node: node.to_untrusted_node_address(),
-                property_name: property_animation.property_name().into(),
-                elapsed_time: property_animation.duration,
+                property_or_animation_name,
+                elapsed_time,
             })
             .unwrap()
     };
 
-    handle_cancelled_animations(animation_state, send_transition_event);
-    handle_running_animations(animation_state, now, send_transition_event);
-    handle_new_animations(animation_state, send_transition_event);
+    handle_cancelled_animations(animation_state, now, send_event);
+    handle_running_animations(animation_state, now, send_event);
+    handle_new_animations(animation_state, send_event);
 }
 
 /// Walk through the list of running animations and remove all of the ones that
@@ -123,7 +125,7 @@ pub fn update_animation_state(
 pub fn handle_running_animations(
     animation_state: &mut ElementAnimationState,
     now: f64,
-    mut send_transition_event: impl FnMut(&PropertyAnimation, TransitionEventType),
+    mut send_event: impl FnMut(&Animation, TransitionOrAnimationEventType, f64),
 ) {
     let mut running_animations =
         std::mem::replace(&mut animation_state.running_animations, Vec::new());
@@ -144,9 +146,18 @@ pub fn handle_running_animations(
             animation_state.running_animations.push(running_animation);
         } else {
             debug!("Finishing transition: {:?}", running_animation);
-            if let Animation::Transition(_, _, ref property_animation) = running_animation {
-                send_transition_event(property_animation, TransitionEventType::TransitionEnd);
-            }
+            let (event_type, elapsed_time) = match running_animation {
+                Animation::Transition(_, _, ref property_animation) => (
+                    TransitionOrAnimationEventType::TransitionEnd,
+                    property_animation.duration,
+                ),
+                Animation::Keyframes(_, _, _, ref mut state) => (
+                    TransitionOrAnimationEventType::AnimationEnd,
+                    state.active_duration(),
+                ),
+            };
+
+            send_event(&running_animation, event_type, elapsed_time);
             animation_state.finished_animations.push(running_animation);
         }
     }
@@ -157,12 +168,19 @@ pub fn handle_running_animations(
 /// well.
 pub fn handle_cancelled_animations(
     animation_state: &mut ElementAnimationState,
-    mut send_transition_event: impl FnMut(&PropertyAnimation, TransitionEventType),
+    now: f64,
+    mut send_event: impl FnMut(&Animation, TransitionOrAnimationEventType, f64),
 ) {
     for animation in animation_state.cancelled_animations.drain(..) {
         match animation {
-            Animation::Transition(_, _, ref property_animation) => {
-                send_transition_event(property_animation, TransitionEventType::TransitionCancel)
+            Animation::Transition(_, start_time, _) => {
+                // TODO(mrobinson): We need to properly compute the elapsed_time here
+                // according to https://drafts.csswg.org/css-transitions/#event-transitionevent
+                send_event(
+                    &animation,
+                    TransitionOrAnimationEventType::TransitionCancel,
+                    (now - start_time).max(0.),
+                );
             },
             // TODO(mrobinson): We should send animationcancel events.
             Animation::Keyframes(..) => {},
@@ -172,12 +190,18 @@ pub fn handle_cancelled_animations(
 
 pub fn handle_new_animations(
     animation_state: &mut ElementAnimationState,
-    mut send_transition_event: impl FnMut(&PropertyAnimation, TransitionEventType),
+    mut send_event: impl FnMut(&Animation, TransitionOrAnimationEventType, f64),
 ) {
     for animation in animation_state.new_animations.drain(..) {
         match animation {
-            Animation::Transition(_, _, ref property_animation) => {
-                send_transition_event(property_animation, TransitionEventType::TransitionRun)
+            Animation::Transition(..) => {
+                // TODO(mrobinson): We need to properly compute the elapsed_time here
+                // according to https://drafts.csswg.org/css-transitions/#event-transitionevent
+                send_event(
+                    &animation,
+                    TransitionOrAnimationEventType::TransitionRun,
+                    0.,
+                )
             },
             Animation::Keyframes(..) => {},
         }
diff --git a/components/layout/context.rs b/components/layout/context.rs
index f98f8bc8395..fc243b8234f 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -86,9 +86,9 @@ pub struct LayoutContext<'a> {
     /// A None value means that this layout was not initiated by the script thread.
     pub pending_images: Option<Mutex<Vec<PendingImage>>>,
 
-    /// A list of nodes that have just initiated a CSS transition.
+    /// A list of nodes that have just initiated a CSS transition or animation.
     /// A None value means that this layout was not initiated by the script thread.
-    pub newly_transitioning_nodes: Option<Mutex<Vec<UntrustedNodeAddress>>>,
+    pub newly_animating_nodes: Option<Mutex<Vec<UntrustedNodeAddress>>>,
 }
 
 impl<'a> Drop for LayoutContext<'a> {
diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index 4a346315e8a..046a9a6f30e 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -648,7 +648,7 @@ impl LayoutThread {
             } else {
                 None
             },
-            newly_transitioning_nodes: if script_initiated_layout {
+            newly_animating_nodes: if script_initiated_layout {
                 Some(Mutex::new(vec![]))
             } else {
                 None
@@ -1565,11 +1565,11 @@ impl LayoutThread {
         };
         reflow_result.pending_images = pending_images;
 
-        let newly_transitioning_nodes = match context.newly_transitioning_nodes {
+        let newly_animating_nodes = match context.newly_animating_nodes {
             Some(ref nodes) => std::mem::replace(&mut *nodes.lock().unwrap(), vec![]),
             None => vec![],
         };
-        reflow_result.newly_transitioning_nodes = newly_transitioning_nodes;
+        reflow_result.newly_animating_nodes = newly_animating_nodes;
 
         let mut root_flow = match self.root_flow.borrow().clone() {
             Some(root_flow) => root_flow,
@@ -1741,7 +1741,7 @@ impl LayoutThread {
                 invalid_nodes,
             );
             assert!(layout_context.pending_images.is_none());
-            assert!(layout_context.newly_transitioning_nodes.is_none());
+            assert!(layout_context.newly_animating_nodes.is_none());
         }
     }
 
@@ -1756,19 +1756,13 @@ impl LayoutThread {
         invalid_nodes: FxHashSet<OpaqueNode>,
     ) {
         {
-            let mut newly_transitioning_nodes = context
-                .newly_transitioning_nodes
+            let mut newly_animating_nodes = context
+                .newly_animating_nodes
                 .as_ref()
                 .map(|nodes| nodes.lock().unwrap());
-            let newly_transitioning_nodes =
-                newly_transitioning_nodes.as_mut().map(|nodes| &mut **nodes);
+            let newly_animating_nodes = newly_animating_nodes.as_mut().map(|nodes| &mut **nodes);
             let mut animation_states = self.animation_states.write();
-
-            animation::collect_newly_transitioning_nodes(
-                &animation_states,
-                newly_transitioning_nodes,
-            );
-
+            animation::collect_newly_animating_nodes(&animation_states, newly_animating_nodes);
             animation::update_animation_states(
                 &self.constellation_chan,
                 &self.script_chan,
diff --git a/components/script/dom/animationevent.rs b/components/script/dom/animationevent.rs
new file mode 100644
index 00000000000..191b460228d
--- /dev/null
+++ b/components/script/dom/animationevent.rs
@@ -0,0 +1,76 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use crate::dom::bindings::codegen::Bindings::AnimationEventBinding::{
+    AnimationEventInit, AnimationEventMethods,
+};
+use crate::dom::bindings::codegen::Bindings::EventBinding::EventMethods;
+use crate::dom::bindings::inheritance::Castable;
+use crate::dom::bindings::num::Finite;
+use crate::dom::bindings::reflector::reflect_dom_object;
+use crate::dom::bindings::root::DomRoot;
+use crate::dom::bindings::str::DOMString;
+use crate::dom::event::Event;
+use crate::dom::window::Window;
+use dom_struct::dom_struct;
+use servo_atoms::Atom;
+
+#[dom_struct]
+pub struct AnimationEvent {
+    event: Event,
+    animation_name: Atom,
+    elapsed_time: Finite<f32>,
+    pseudo_element: DOMString,
+}
+
+impl AnimationEvent {
+    fn new_inherited(init: &AnimationEventInit) -> AnimationEvent {
+        AnimationEvent {
+            event: Event::new_inherited(),
+            animation_name: Atom::from(init.animationName.clone()),
+            elapsed_time: init.elapsedTime.clone(),
+            pseudo_element: init.pseudoElement.clone(),
+        }
+    }
+
+    pub fn new(window: &Window, type_: Atom, init: &AnimationEventInit) -> DomRoot<AnimationEvent> {
+        let ev = reflect_dom_object(Box::new(AnimationEvent::new_inherited(init)), window);
+        {
+            let event = ev.upcast::<Event>();
+            event.init_event(type_, init.parent.bubbles, init.parent.cancelable);
+        }
+        ev
+    }
+
+    #[allow(non_snake_case)]
+    pub fn Constructor(
+        window: &Window,
+        type_: DOMString,
+        init: &AnimationEventInit,
+    ) -> DomRoot<AnimationEvent> {
+        AnimationEvent::new(window, Atom::from(type_), init)
+    }
+}
+
+impl AnimationEventMethods for AnimationEvent {
+    // https://drafts.csswg.org/css-animations/#interface-animationevent-attributes
+    fn AnimationName(&self) -> DOMString {
+        DOMString::from(&*self.animation_name)
+    }
+
+    // https://drafts.csswg.org/css-animations/#interface-animationevent-attributes
+    fn ElapsedTime(&self) -> Finite<f32> {
+        self.elapsed_time.clone()
+    }
+
+    // https://drafts.csswg.org/css-animations/#interface-animationevent-attributes
+    fn PseudoElement(&self) -> DOMString {
+        self.pseudo_element.clone()
+    }
+
+    // https://dom.spec.whatwg.org/#dom-event-istrusted
+    fn IsTrusted(&self) -> bool {
+        self.upcast::<Event>().IsTrusted()
+    }
+}
diff --git a/components/script/dom/macros.rs b/components/script/dom/macros.rs
index b1ea4f5cde2..871e831dc82 100644
--- a/components/script/dom/macros.rs
+++ b/components/script/dom/macros.rs
@@ -442,6 +442,7 @@ macro_rules! global_event_handlers(
     );
     (NoOnload) => (
         event_handler!(abort, GetOnabort, SetOnabort);
+        event_handler!(animationend, GetOnanimationend, SetOnanimationend);
         event_handler!(cancel, GetOncancel, SetOncancel);
         event_handler!(canplay, GetOncanplay, SetOncanplay);
         event_handler!(canplaythrough, GetOncanplaythrough, SetOncanplaythrough);
diff --git a/components/script/dom/mod.rs b/components/script/dom/mod.rs
index a0f6241d55e..9536f113417 100644
--- a/components/script/dom/mod.rs
+++ b/components/script/dom/mod.rs
@@ -213,6 +213,7 @@ pub mod abstractworker;
 pub mod abstractworkerglobalscope;
 pub mod activation;
 pub mod analysernode;
+pub mod animationevent;
 pub mod attr;
 pub mod audiobuffer;
 pub mod audiobuffersourcenode;
diff --git a/components/script/dom/webidls/AnimationEvent.webidl b/components/script/dom/webidls/AnimationEvent.webidl
new file mode 100644
index 00000000000..fd9d6c47f7e
--- /dev/null
+++ b/components/script/dom/webidls/AnimationEvent.webidl
@@ -0,0 +1,26 @@
+/* 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
+ * http://www.w3.org/TR/css3-animations/#animation-events-
+ * http://dev.w3.org/csswg/css3-animations/#animation-events-
+ *
+ * Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C
+ * liability, trademark and document use rules apply.
+ */
+
+[Exposed=Window]
+interface AnimationEvent : Event {
+  constructor(DOMString type, optional AnimationEventInit eventInitDict = {});
+
+  readonly attribute DOMString animationName;
+  readonly attribute float     elapsedTime;
+  readonly attribute DOMString pseudoElement;
+};
+
+dictionary AnimationEventInit : EventInit {
+  DOMString animationName = "";
+  float elapsedTime = 0;
+  DOMString pseudoElement = "";
+};
diff --git a/components/script/dom/webidls/EventHandler.webidl b/components/script/dom/webidls/EventHandler.webidl
index a3514182806..b5617564fbc 100644
--- a/components/script/dom/webidls/EventHandler.webidl
+++ b/components/script/dom/webidls/EventHandler.webidl
@@ -90,6 +90,11 @@ interface mixin GlobalEventHandlers {
            attribute EventHandler onwaiting;
 };
 
+// https://drafts.csswg.org/css-animations/#interface-globaleventhandlers-idl
+partial interface mixin GlobalEventHandlers {
+           attribute EventHandler onanimationend;
+};
+
 // https://drafts.csswg.org/css-transitions/#interface-globaleventhandlers-idl
 partial interface mixin GlobalEventHandlers {
            attribute EventHandler ontransitionrun;
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index e6d8c1bd13f..ccb4b8dfb48 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -1582,11 +1582,11 @@ impl Window {
         }
 
         let for_display = reflow_goal == ReflowGoal::Full;
+        let pipeline_id = self.upcast::<GlobalScope>().pipeline_id();
         if for_display && self.suppress_reflow.get() {
             debug!(
                 "Suppressing reflow pipeline {} for reason {:?} before FirstLoad or RefreshTick",
-                self.upcast::<GlobalScope>().pipeline_id(),
-                reason
+                pipeline_id, reason
             );
             return false;
         }
@@ -1617,11 +1617,7 @@ impl Window {
 
         // On debug mode, print the reflow event information.
         if self.relayout_event {
-            debug_reflow_events(
-                self.upcast::<GlobalScope>().pipeline_id(),
-                &reflow_goal,
-                &reason,
-            );
+            debug_reflow_events(pipeline_id, &reflow_goal, &reason);
         }
 
         let document = self.Document();
@@ -1699,12 +1695,11 @@ impl Window {
             {
                 let (responder, responder_listener) =
                     ProfiledIpc::channel(self.global().time_profiler_chan().clone()).unwrap();
-                let pipeline = self.upcast::<GlobalScope>().pipeline_id();
                 let image_cache_chan = self.image_cache_chan.clone();
                 ROUTER.add_route(
                     responder_listener.to_opaque(),
                     Box::new(move |message| {
-                        let _ = image_cache_chan.send((pipeline, message.to().unwrap()));
+                        let _ = image_cache_chan.send((pipeline_id, message.to().unwrap()));
                     }),
                 );
                 self.image_cache
@@ -1714,7 +1709,7 @@ impl Window {
         }
 
         unsafe {
-            ScriptThread::note_newly_transitioning_nodes(complete.newly_transitioning_nodes);
+            ScriptThread::note_newly_animating_nodes(pipeline_id, complete.newly_animating_nodes);
         }
 
         true
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 16fd866f1b9..165e526fa76 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -19,7 +19,9 @@
 
 use crate::devtools;
 use crate::document_loader::DocumentLoader;
+use crate::dom::animationevent::AnimationEvent;
 use crate::dom::bindings::cell::DomRefCell;
+use crate::dom::bindings::codegen::Bindings::AnimationEventBinding::AnimationEventInit;
 use crate::dom::bindings::codegen::Bindings::DocumentBinding::{
     DocumentMethods, DocumentReadyState,
 };
@@ -138,9 +140,9 @@ use script_traits::{
     EventResult, HistoryEntryReplacement, InitialScriptState, JsEvalResult, LayoutMsg, LoadData,
     LoadOrigin, MediaSessionActionType, MouseButton, MouseEventType, NewLayoutInfo, Painter,
     ProgressiveWebMetricType, ScriptMsg, ScriptThreadFactory, ScriptToConstellationChan,
-    StructuredSerializedData, TimerSchedulerMsg, TouchEventType, TouchId, TransitionEventType,
-    UntrustedNodeAddress, UpdatePipelineIdReason, WebrenderIpcSender, WheelDelta, WindowSizeData,
-    WindowSizeType,
+    StructuredSerializedData, TimerSchedulerMsg, TouchEventType, TouchId,
+    TransitionOrAnimationEventType, UntrustedNodeAddress, UpdatePipelineIdReason,
+    WebrenderIpcSender, WheelDelta, WindowSizeData, WindowSizeType,
 };
 use servo_atoms::Atom;
 use servo_config::opts;
@@ -639,7 +641,7 @@ pub struct ScriptThread {
 
     /// A list of nodes with in-progress CSS transitions, which roots them for the duration
     /// of the transition.
-    transitioning_nodes: DomRefCell<Vec<Dom<Node>>>,
+    animating_nodes: DomRefCell<HashMap<PipelineId, Vec<Dom<Node>>>>,
 
     /// <https://html.spec.whatwg.org/multipage/#custom-element-reactions-stack>
     custom_element_reaction_stack: CustomElementReactionStack,
@@ -823,7 +825,10 @@ impl ScriptThread {
         })
     }
 
-    pub unsafe fn note_newly_transitioning_nodes(nodes: Vec<UntrustedNodeAddress>) {
+    pub unsafe fn note_newly_animating_nodes(
+        pipeline_id: PipelineId,
+        nodes: Vec<UntrustedNodeAddress>,
+    ) {
         SCRIPT_THREAD_ROOT.with(|root| {
             let script_thread = &*root.get().unwrap();
             let js_runtime = script_thread.js_runtime.rt();
@@ -831,8 +836,10 @@ impl ScriptThread {
                 .into_iter()
                 .map(|n| Dom::from_ref(&*from_untrusted_node_address(js_runtime, n)));
             script_thread
-                .transitioning_nodes
+                .animating_nodes
                 .borrow_mut()
+                .entry(pipeline_id)
+                .or_insert_with(Vec::new)
                 .extend(new_nodes);
         })
     }
@@ -1345,7 +1352,7 @@ impl ScriptThread {
 
             docs_with_no_blocking_loads: Default::default(),
 
-            transitioning_nodes: Default::default(),
+            animating_nodes: Default::default(),
 
             custom_element_reaction_stack: CustomElementReactionStack::new(),
 
@@ -1697,7 +1704,7 @@ impl ScriptThread {
                 FocusIFrame(id, ..) => Some(id),
                 WebDriverScriptCommand(id, ..) => Some(id),
                 TickAllAnimations(id) => Some(id),
-                TransitionEvent { .. } => None,
+                TransitionOrAnimationEvent { .. } => None,
                 WebFontLoaded(id) => Some(id),
                 DispatchIFrameLoadEvent {
                     target: _,
@@ -1898,18 +1905,18 @@ impl ScriptThread {
             ConstellationControlMsg::TickAllAnimations(pipeline_id) => {
                 self.handle_tick_all_animations(pipeline_id)
             },
-            ConstellationControlMsg::TransitionEvent {
+            ConstellationControlMsg::TransitionOrAnimationEvent {
                 pipeline_id,
                 event_type,
                 node,
-                property_name,
+                property_or_animation_name,
                 elapsed_time,
             } => {
-                self.handle_transition_event(
+                self.handle_transition_or_animation_event(
                     pipeline_id,
                     event_type,
                     node,
-                    property_name,
+                    property_or_animation_name,
                     elapsed_time,
                 );
             },
@@ -2846,6 +2853,9 @@ impl ScriptThread {
             .send((id, ScriptMsg::PipelineExited))
             .ok();
 
+        // Remove any rooted nodes for active animations and transitions.
+        self.animating_nodes.borrow_mut().remove(&id);
+
         // Now that layout is shut down, it's OK to remove the document.
         if let Some(document) = document {
             // We don't want to dispatch `mouseout` event pointing to non-existing element
@@ -2916,66 +2926,91 @@ impl ScriptThread {
     /// Handles firing of transition-related events.
     ///
     /// TODO(mrobinson): Add support for more events.
-    fn handle_transition_event(
+    fn handle_transition_or_animation_event(
         &self,
         pipeline_id: PipelineId,
-        event_type: TransitionEventType,
+        event_type: TransitionOrAnimationEventType,
         unsafe_node: UntrustedNodeAddress,
-        property_name: String,
+        property_or_animation_name: String,
         elapsed_time: f64,
     ) {
         let js_runtime = self.js_runtime.rt();
         let node = unsafe { from_untrusted_node_address(js_runtime, unsafe_node) };
 
-        let node_index = self
-            .transitioning_nodes
-            .borrow()
-            .iter()
-            .position(|n| &**n as *const _ == &*node as *const _);
-        let node_index = match node_index {
-            Some(node_index) => node_index,
-            None => {
-                // If no index is found, we can't know whether this node is safe to use.
-                // It's better not to fire a DOM event than crash.
-                warn!("Ignoring transition end notification for unknown node.");
-                return;
-            },
-        };
+        // We limit the scope of the borrow here, so that we don't maintain this borrow
+        // and then incidentally trigger another layout. That might result in a double
+        // mutable borrow of `animating_nodes`.
+        {
+            let mut animating_nodes = self.animating_nodes.borrow_mut();
+            let nodes = match animating_nodes.get_mut(&pipeline_id) {
+                Some(nodes) => nodes,
+                None => {
+                    return warn!(
+                        "Ignoring transition event for pipeline without animating nodes."
+                    );
+                },
+            };
 
-        if self.closed_pipelines.borrow().contains(&pipeline_id) {
-            warn!("Ignoring transition event for closed pipeline.");
-            return;
+            let node_index = nodes
+                .iter()
+                .position(|n| &**n as *const _ == &*node as *const _);
+            let node_index = match node_index {
+                Some(node_index) => node_index,
+                None => {
+                    // If no index is found, we can't know whether this node is safe to use.
+                    // It's better not to fire a DOM event than crash.
+                    warn!("Ignoring transition event for unknown node.");
+                    return;
+                },
+            };
+
+            if event_type.finalizes_transition_or_animation() {
+                nodes.remove(node_index);
+            }
+        }
+
+        // Not quite the right thing - see #13865.
+        if event_type.finalizes_transition_or_animation() {
+            node.dirty(NodeDamage::NodeStyleDamaged);
         }
 
         let event_atom = match event_type {
-            TransitionEventType::TransitionRun => atom!("transitionrun"),
-            TransitionEventType::TransitionEnd => {
-                // Not quite the right thing - see #13865.
-                node.dirty(NodeDamage::NodeStyleDamaged);
-                self.transitioning_nodes.borrow_mut().remove(node_index);
-                atom!("transitionend")
-            },
-            TransitionEventType::TransitionCancel => {
-                self.transitioning_nodes.borrow_mut().remove(node_index);
-                atom!("transitioncancel")
-            },
+            TransitionOrAnimationEventType::AnimationEnd => atom!("animationend"),
+            TransitionOrAnimationEventType::TransitionCancel => atom!("transitioncancel"),
+            TransitionOrAnimationEventType::TransitionEnd => atom!("transitionend"),
+            TransitionOrAnimationEventType::TransitionRun => atom!("transitionrun"),
         };
-
-        let event_init = TransitionEventInit {
-            parent: EventInit {
-                bubbles: true,
-                cancelable: false,
-            },
-            propertyName: DOMString::from(property_name),
-            elapsedTime: Finite::new(elapsed_time as f32).unwrap(),
-            // TODO: Handle pseudo-elements properly
-            pseudoElement: DOMString::new(),
+        let parent = EventInit {
+            bubbles: true,
+            cancelable: false,
         };
 
+        // TODO: Handle pseudo-elements properly
+        let property_or_animation_name = DOMString::from(property_or_animation_name);
+        let elapsed_time = Finite::new(elapsed_time as f32).unwrap();
         let window = window_from_node(&*node);
-        TransitionEvent::new(&window, event_atom, &event_init)
-            .upcast::<Event>()
-            .fire(node.upcast());
+
+        if event_type.is_transition_event() {
+            let event_init = TransitionEventInit {
+                parent,
+                propertyName: property_or_animation_name,
+                elapsedTime: elapsed_time,
+                pseudoElement: DOMString::new(),
+            };
+            TransitionEvent::new(&window, event_atom, &event_init)
+                .upcast::<Event>()
+                .fire(node.upcast());
+        } else {
+            let event_init = AnimationEventInit {
+                parent,
+                animationName: property_or_animation_name,
+                elapsedTime: elapsed_time,
+                pseudoElement: DOMString::new(),
+            };
+            AnimationEvent::new(&window, event_atom, &event_init)
+                .upcast::<Event>()
+                .fire(node.upcast());
+        }
     }
 
     /// Handles a Web font being loaded. Does nothing if the page no longer exists.
diff --git a/components/script_layout_interface/message.rs b/components/script_layout_interface/message.rs
index 63854134cac..fe117436b51 100644
--- a/components/script_layout_interface/message.rs
+++ b/components/script_layout_interface/message.rs
@@ -193,7 +193,7 @@ pub struct ReflowComplete {
     /// The list of images that were encountered that are in progress.
     pub pending_images: Vec<PendingImage>,
     /// The list of nodes that initiated a CSS transition.
-    pub newly_transitioning_nodes: Vec<UntrustedNodeAddress>,
+    pub newly_animating_nodes: Vec<UntrustedNodeAddress>,
 }
 
 /// Information needed for a script-initiated reflow.
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index 3db883b6536..c23221f929f 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -282,16 +282,40 @@ pub enum UpdatePipelineIdReason {
     Traversal,
 }
 
-/// The type of transition event to trigger.
+/// The type of transition event to trigger. These are defined by
+/// CSS Transitions § 6.1 and CSS Animations § 4.2
 #[derive(Clone, Debug, Deserialize, Serialize)]
-pub enum TransitionEventType {
-    /// The transition has started running.
+pub enum TransitionOrAnimationEventType {
+    /// "The transitionrun event occurs when a transition is created (i.e., when it
+    /// is added to the set of running transitions)."
     TransitionRun,
-    /// The transition has ended by reaching the end of its animation.
+    /// "The transitionend event occurs at the completion of the transition. In the
+    /// case where a transition is removed before completion, such as if the
+    /// transition-property is removed, then the event will not fire."
     TransitionEnd,
-    /// The transition ended early for some reason, such as the property
-    /// no longer being transitionable or being replaced by another transition.
+    /// "The transitioncancel event occurs when a transition is canceled."
     TransitionCancel,
+    /// "The animationend event occurs when the animation finishes"
+    AnimationEnd,
+}
+
+impl TransitionOrAnimationEventType {
+    /// Whether or not this event finalizes the animation or transition. During finalization
+    /// the DOM object associated with this transition or animation is unrooted.
+    pub fn finalizes_transition_or_animation(&self) -> bool {
+        match *self {
+            Self::TransitionEnd | Self::TransitionCancel | Self::AnimationEnd => true,
+            Self::TransitionRun => false,
+        }
+    }
+
+    /// Whether or not this event is a transition-related event.
+    pub fn is_transition_event(&self) -> bool {
+        match *self {
+            Self::TransitionRun | Self::TransitionEnd | Self::TransitionCancel => true,
+            Self::AnimationEnd => false,
+        }
+    }
 }
 
 /// Messages sent from the constellation or layout to the script thread.
@@ -380,16 +404,17 @@ pub enum ConstellationControlMsg {
     WebDriverScriptCommand(PipelineId, WebDriverScriptCommand),
     /// Notifies script thread that all animations are done
     TickAllAnimations(PipelineId),
-    /// Notifies the script thread that a transition related event should be sent.
-    TransitionEvent {
+    /// Notifies the script thread that a transition or animation related event should be sent.
+    TransitionOrAnimationEvent {
         /// The pipeline id of the layout task that sent this message.
         pipeline_id: PipelineId,
         /// The type of transition event this should trigger.
-        event_type: TransitionEventType,
+        event_type: TransitionOrAnimationEventType,
         /// The address of the node which owns this transition.
         node: UntrustedNodeAddress,
-        /// The property name of the property that is transitioning.
-        property_name: String,
+        /// The name of the property that is transitioning (in the case of a transition)
+        /// or the name of the animation (in the case of an animation).
+        property_or_animation_name: String,
         /// The elapsed time property to send with this transition event.
         elapsed_time: f64,
     },
@@ -452,7 +477,7 @@ impl fmt::Debug for ConstellationControlMsg {
             FocusIFrame(..) => "FocusIFrame",
             WebDriverScriptCommand(..) => "WebDriverScriptCommand",
             TickAllAnimations(..) => "TickAllAnimations",
-            TransitionEvent { .. } => "TransitionEvent",
+            TransitionOrAnimationEvent { .. } => "TransitionOrAnimationEvent",
             WebFontLoaded(..) => "WebFontLoaded",
             DispatchIFrameLoadEvent { .. } => "DispatchIFrameLoadEvent",
             DispatchStorageEvent(..) => "DispatchStorageEvent",
diff --git a/components/style/animation.rs b/components/style/animation.rs
index 59c9f847e96..eed3aaada1a 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -172,6 +172,17 @@ impl KeyframesAnimationState {
         self.current_direction = old_direction;
         self.started_at = new_started_at;
     }
+
+    /// Calculate the active-duration of this animation according to
+    /// https://drafts.csswg.org/css-animations/#active-duration. active-duration
+    /// is not really meaningful for infinite animations so we just return 0
+    /// here in that case.
+    pub fn active_duration(&self) -> f64 {
+        match self.iteration_state {
+            KeyframesIterationState::Finite(_, max) => self.duration * (max as f64),
+            KeyframesIterationState::Infinite => 0.,
+        }
+    }
 }
 
 impl fmt::Debug for KeyframesAnimationState {
diff --git a/tests/wpt/metadata/css/css-animations/Element-getAnimations.tentative.html.ini b/tests/wpt/metadata/css/css-animations/Element-getAnimations.tentative.html.ini
index bf27b93175b..854c8c6e480 100644
--- a/tests/wpt/metadata/css/css-animations/Element-getAnimations.tentative.html.ini
+++ b/tests/wpt/metadata/css/css-animations/Element-getAnimations.tentative.html.ini
@@ -1,6 +1,5 @@
 [Element-getAnimations.tentative.html]
   bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
   [getAnimations for CSS Animations with animation-name: none]
     expected: FAIL
 
@@ -35,7 +34,7 @@
     expected: FAIL
 
   [getAnimations for CSS Animations that have finished but are forwards filling]
-    expected: TIMEOUT
+    expected: FAIL
 
   [getAnimations returns objects with the same identity]
     expected: FAIL
@@ -56,7 +55,7 @@
     expected: FAIL
 
   [getAnimations for CSS Animations that have finished]
-    expected: TIMEOUT
+    expected: FAIL
 
   [{ subtree: false } on a leaf element returns the element's animations and ignore pseudo-elements]
     expected: FAIL
diff --git a/tests/wpt/metadata/css/css-animations/animationevent-interface.html.ini b/tests/wpt/metadata/css/css-animations/animationevent-interface.html.ini
deleted file mode 100644
index a4d160df59e..00000000000
--- a/tests/wpt/metadata/css/css-animations/animationevent-interface.html.ini
+++ /dev/null
@@ -1,124 +0,0 @@
-[animationevent-interface.html]
-  [the event is an instance of AnimationEvent]
-    expected: FAIL
-
-  [animationName is readonly]
-    expected: FAIL
-
-  [type argument is null]
-    expected: FAIL
-
-  [elapsedTime has default value of 0.0]
-    expected: FAIL
-
-  [elapsedTime is readonly]
-    expected: FAIL
-
-  [elapsedTime set to 0.5]
-    expected: FAIL
-
-  [animationEventInit argument is empty dictionary]
-    expected: FAIL
-
-  [event type set to undefined]
-    expected: FAIL
-
-  [Missing type argument]
-    expected: FAIL
-
-  [AnimationEvent.pseudoElement initialized from the dictionary]
-    expected: FAIL
-
-  [the event inherts from Event]
-    expected: FAIL
-
-  [type argument is string]
-    expected: FAIL
-
-  [animationName set to 'sample']
-    expected: FAIL
-
-  [animationEventInit argument is undefined]
-    expected: FAIL
-
-  [animationEventInit argument is null]
-    expected: FAIL
-
-  [AnimationEventInit properties set value]
-    expected: FAIL
-
-  [animationName has default value of empty string]
-    expected: FAIL
-
-  [elapsedTime set to -0.5]
-    expected: FAIL
-
-  [elapsedTime cannot be set to -Infinity]
-    expected: FAIL
-
-  [elapsedTime cannot be set to Infinity]
-    expected: FAIL
-
-  [animationName set to [\]]
-    expected: FAIL
-
-  [elapsedTime set to null]
-    expected: FAIL
-
-  [elapsedTime set to an object with a valueOf function]
-    expected: FAIL
-
-  [animationName set to an object with a valueOf function]
-    expected: FAIL
-
-  [elapsedTime cannot be set to NaN]
-    expected: FAIL
-
-  [elapsedTime set to true]
-    expected: FAIL
-
-  [elapsedTime set to '']
-    expected: FAIL
-
-  [elapsedTime set to [0.5\]]
-    expected: FAIL
-
-  [elapsedTime set to undefined]
-    expected: FAIL
-
-  [animationName set to true]
-    expected: FAIL
-
-  [elapsedTime set to false]
-    expected: FAIL
-
-  [elapsedTime set to [\]]
-    expected: FAIL
-
-  [elapsedTime cannot be set to [0.5, 1.0\]]
-    expected: FAIL
-
-  [animationName set to null]
-    expected: FAIL
-
-  [animationName set to an object]
-    expected: FAIL
-
-  [elapsedTime cannot be set to 'sample']
-    expected: FAIL
-
-  [animationName set to undefined]
-    expected: FAIL
-
-  [animationName set to [1, 2, 3\]]
-    expected: FAIL
-
-  [animationName set to false]
-    expected: FAIL
-
-  [animationName set to a number]
-    expected: FAIL
-
-  [elapsedTime cannot be set to an object]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/css-animations/animationevent-types.html.ini b/tests/wpt/metadata/css/css-animations/animationevent-types.html.ini
index 9a5d66fd1ef..70ed3998119 100644
--- a/tests/wpt/metadata/css/css-animations/animationevent-types.html.ini
+++ b/tests/wpt/metadata/css/css-animations/animationevent-types.html.ini
@@ -4,9 +4,6 @@
   [animationiteration event is instanceof AnimationEvent]
     expected: TIMEOUT
 
-  [animationend event is instanceof AnimationEvent]
-    expected: TIMEOUT
-
   [animationstart event is instanceof AnimationEvent]
     expected: TIMEOUT
 
diff --git a/tests/wpt/metadata/css/css-animations/idlharness.html.ini b/tests/wpt/metadata/css/css-animations/idlharness.html.ini
index 01e1d42bf7f..6b8f7e14289 100644
--- a/tests/wpt/metadata/css/css-animations/idlharness.html.ini
+++ b/tests/wpt/metadata/css/css-animations/idlharness.html.ini
@@ -1,61 +1,25 @@
 [idlharness.html]
-  [AnimationEvent interface: attribute pseudoElement]
-    expected: FAIL
-
-  [AnimationEvent interface: existence and properties of interface prototype object]
-    expected: FAIL
-
-  [Window interface: attribute onanimationend]
-    expected: FAIL
-
   [Document interface: attribute onanimationiteration]
     expected: FAIL
 
-  [AnimationEvent interface object length]
-    expected: FAIL
-
   [CSSKeyframeRule interface: attribute style]
     expected: FAIL
 
-  [AnimationEvent interface object name]
-    expected: FAIL
-
-  [AnimationEvent interface: attribute elapsedTime]
-    expected: FAIL
-
   [Document interface: attribute onanimationstart]
     expected: FAIL
 
   [HTMLElement interface: attribute onanimationiteration]
     expected: FAIL
 
-  [AnimationEvent must be primary interface of new AnimationEvent("animationstart")]
-    expected: FAIL
-
-  [AnimationEvent interface: existence and properties of interface prototype object's @@unscopables property]
-    expected: FAIL
-
   [Window interface: attribute onanimationcancel]
     expected: FAIL
 
   [CSSKeyframeRule interface: attribute keyText]
     expected: FAIL
 
-  [AnimationEvent interface: new AnimationEvent("animationstart") must inherit property "pseudoElement" with the proper type]
-    expected: FAIL
-
-  [Stringification of new AnimationEvent("animationstart")]
-    expected: FAIL
-
-  [Document interface: attribute onanimationend]
-    expected: FAIL
-
   [HTMLElement interface: attribute onanimationstart]
     expected: FAIL
 
-  [AnimationEvent interface: new AnimationEvent("animationstart") must inherit property "animationName" with the proper type]
-    expected: FAIL
-
   [Document interface: attribute onanimationcancel]
     expected: FAIL
 
@@ -65,24 +29,9 @@
   [HTMLElement interface: attribute onanimationcancel]
     expected: FAIL
 
-  [AnimationEvent interface: attribute animationName]
-    expected: FAIL
-
   [Window interface: attribute onanimationiteration]
     expected: FAIL
 
-  [AnimationEvent interface: new AnimationEvent("animationstart") must inherit property "elapsedTime" with the proper type]
-    expected: FAIL
-
   [CSSKeyframeRule interface: keyframes.cssRules[0\] must inherit property "keyText" with the proper type]
     expected: FAIL
 
-  [AnimationEvent interface: existence and properties of interface prototype object's "constructor" property]
-    expected: FAIL
-
-  [AnimationEvent interface: existence and properties of interface object]
-    expected: FAIL
-
-  [HTMLElement interface: attribute onanimationend]
-    expected: FAIL
-
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-shorthand.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-shorthand.html.ini
index baa566691e6..5d6b41c4332 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-shorthand.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-shorthand.html.ini
@@ -1,9 +1,8 @@
 [variable-animation-substitute-into-keyframe-shorthand.html]
   bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
   [Verify border-bottom-color before animation]
     expected: FAIL
 
   [Verify border-bottom-color after animation]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-transform.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-transform.html.ini
index 83d7114a33c..77ff1af8a7d 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-transform.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe-transform.html.ini
@@ -1,9 +1,8 @@
 [variable-animation-substitute-into-keyframe-transform.html]
   bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
   [Verify transform before animation]
     expected: FAIL
 
   [Verify transform after animation]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe.html.ini
index 1e19344cbec..225a08b98ac 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-into-keyframe.html.ini
@@ -1,9 +1,8 @@
 [variable-animation-substitute-into-keyframe.html]
   bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
   [Verify color before animation]
     expected: FAIL
 
   [Verify color after animation]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-fallback.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-fallback.html.ini
index ec903a327a8..a0f8055196a 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-fallback.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-fallback.html.ini
@@ -1,9 +1,8 @@
 [variable-animation-substitute-within-keyframe-fallback.html]
   bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
   [Verify color before animation]
     expected: FAIL
 
   [Verify color after animation]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-multiple.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-multiple.html.ini
index d4f2fefef86..baa8d60195e 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-multiple.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe-multiple.html.ini
@@ -1,9 +1,8 @@
 [variable-animation-substitute-within-keyframe-multiple.html]
   bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
   [Verify color before animation]
     expected: FAIL
 
   [Verify color after animation]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe.html.ini
index 6d7eafff16b..e1e48b487e4 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-substitute-within-keyframe.html.ini
@@ -1,9 +1,8 @@
 [variable-animation-substitute-within-keyframe.html]
   bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
   [Verify color before animation]
     expected: FAIL
 
   [Verify color after animation]
-    expected: TIMEOUT
+    expected: FAIL
 
diff --git a/tests/wpt/metadata/dom/events/webkit-animation-end-event.html.ini b/tests/wpt/metadata/dom/events/webkit-animation-end-event.html.ini
index c560f81d9b6..2b8b4ec6d09 100644
--- a/tests/wpt/metadata/dom/events/webkit-animation-end-event.html.ini
+++ b/tests/wpt/metadata/dom/events/webkit-animation-end-event.html.ini
@@ -1,4 +1,5 @@
 [webkit-animation-end-event.html]
+  expected: TIMEOUT
   [event types for prefixed and unprefixed animationend event handlers should be named appropriately]
     expected: FAIL
 
@@ -6,13 +7,13 @@
     expected: FAIL
 
   [webkitAnimationEnd event listener should not trigger if an unprefixed event handler also exists]
-    expected: FAIL
+    expected: NOTRUN
 
   [webkitAnimationEnd event listener is case sensitive]
-    expected: FAIL
+    expected: NOTRUN
 
   [webkitAnimationEnd event listener should trigger for an animation]
-    expected: FAIL
+    expected: TIMEOUT
 
   [onwebkitanimationend event handler should trigger for an animation]
     expected: FAIL
@@ -20,20 +21,17 @@
   [onanimationend and onwebkitanimationend are not aliases]
     expected: FAIL
 
-  [dispatchEvent of a webkitAnimationEnd event does not trigger an unprefixed event handler or listener]
-    expected: FAIL
-
   [dispatchEvent of a webkitAnimationEnd event does trigger a prefixed event handler or listener]
     expected: FAIL
 
   [webkitAnimationEnd event listener should not trigger if an unprefixed listener also exists]
-    expected: FAIL
+    expected: NOTRUN
 
   [dispatchEvent of an animationend event does not trigger a prefixed event handler or listener]
     expected: FAIL
 
   [event types for prefixed and unprefixed animationend event listeners should be named appropriately]
-    expected: FAIL
+    expected: NOTRUN
 
   [onwebkitanimationend event handler should not trigger if an unprefixed event handler also exists]
     expected: FAIL
diff --git a/tests/wpt/metadata/dom/events/webkit-transition-end-event.html.ini b/tests/wpt/metadata/dom/events/webkit-transition-end-event.html.ini
index 2e7d6443e31..4ee5fc21ce9 100644
--- a/tests/wpt/metadata/dom/events/webkit-transition-end-event.html.ini
+++ b/tests/wpt/metadata/dom/events/webkit-transition-end-event.html.ini
@@ -1,7 +1,4 @@
 [webkit-transition-end-event.html]
-  [dispatchEvent of a webkitTransitionEnd event does not trigger an unprefixed event handler or listener]
-    expected: FAIL
-
   [dispatchEvent of an transitionend event does not trigger a prefixed event handler or listener]
     expected: FAIL
 
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index c4e49f39a76..6ad65fa2d0f 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -13870,7 +13870,7 @@
      ]
     ],
     "interfaces.html": [
-     "1a579837cc22d31a7792566615d9e321b3d7fe39",
+     "b6034be26af3c2edd1ef41703857fa99bd2cd639",
      [
       null,
       {}
diff --git a/tests/wpt/mozilla/tests/mozilla/interfaces.html b/tests/wpt/mozilla/tests/mozilla/interfaces.html
index 1a579837cc2..b6034be26af 100644
--- a/tests/wpt/mozilla/tests/mozilla/interfaces.html
+++ b/tests/wpt/mozilla/tests/mozilla/interfaces.html
@@ -12,6 +12,7 @@
 // IMPORTANT: Do not change the list below without review from a DOM peer!
 test_interfaces([
   "AnalyserNode",
+  "AnimationEvent",
   "Attr",
   "Audio",
   "AudioBuffer",