Move most animation processing to script

This is preparation for sharing this code with layout_2020 and
implementing selective off-the-main-thread animations.

We still look for nodes not in the flow tree in the layout thread.

21 files changed, 444 insertions, 371 deletions

diff --git a/Cargo.lock b/Cargo.lock
index fab3ceb8e28..0f5502f8c31 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4562,6 +4562,7 @@ dependencies = [
  "enum-iterator",
  "euclid",
  "fnv",
+ "fxhash",
  "headers",
  "html5ever",
  "http",
@@ -4641,6 +4642,7 @@ dependencies = [
  "canvas_traits",
  "crossbeam-channel",
  "euclid",
+ "fxhash",
  "gfx_traits",
  "html5ever",
  "ipc-channel",
@@ -4650,6 +4652,7 @@ dependencies = [
  "metrics",
  "msg",
  "net_traits",
+ "parking_lot",
  "profile_traits",
  "range",
  "script_traits",
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index 00dcd2a4eb8..423314e6b0a 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -2237,9 +2237,6 @@ where
     fn handle_request_from_layout(&mut self, message: FromLayoutMsg) {
         debug!("Constellation got {:?} message", message);
         match message {
-            FromLayoutMsg::ChangeRunningAnimationsState(pipeline_id, animation_state) => {
-                self.handle_change_running_animations_state(pipeline_id, animation_state)
-            },
             // Layout sends new sizes for all subframes. This needs to be reflected by all
             // frame trees in the navigation context containing the subframe.
             FromLayoutMsg::IFrameSizes(iframe_sizes) => {
diff --git a/components/layout/animation.rs b/components/layout/animation.rs
deleted file mode 100644
index a331b3b3bbc..00000000000
--- a/components/layout/animation.rs
+++ /dev/null
@@ -1,218 +0,0 @@
-/* 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/. */
-
-//! CSS transitions and animations.
-
-use crate::display_list::items::OpaqueNode;
-use crate::flow::{Flow, GetBaseFlow};
-use crate::opaque_node::OpaqueNodeMethods;
-use fxhash::{FxHashMap, FxHashSet};
-use ipc_channel::ipc::IpcSender;
-use msg::constellation_msg::PipelineId;
-use script_traits::UntrustedNodeAddress;
-use script_traits::{
-    AnimationState as AnimationsPresentState, ConstellationControlMsg,
-    LayoutMsg as ConstellationMsg, TransitionOrAnimationEvent, TransitionOrAnimationEventType,
-};
-use style::animation::{AnimationState, ElementAnimationSet};
-
-/// Processes any new animations that were discovered after style recalculation and
-/// remove animations for any disconnected nodes. Send messages that trigger events
-/// for any events that changed state.
-pub fn do_post_style_animations_update(
-    constellation_chan: &IpcSender<ConstellationMsg>,
-    script_chan: &IpcSender<ConstellationControlMsg>,
-    animation_states: &mut FxHashMap<OpaqueNode, ElementAnimationSet>,
-    pipeline_id: PipelineId,
-    now: f64,
-    out: &mut Vec<UntrustedNodeAddress>,
-    root_flow: &mut dyn Flow,
-) {
-    let had_running_animations = animation_states
-        .values()
-        .any(|state| state.needs_animation_ticks());
-
-    cancel_animations_for_disconnected_nodes(animation_states, root_flow);
-    collect_newly_animating_nodes(animation_states, out);
-
-    for (node, animation_state) in animation_states.iter_mut() {
-        update_animation_state(script_chan, animation_state, pipeline_id, now, *node);
-    }
-
-    // Remove empty states from our collection of states in order to free
-    // up space as soon as we are no longer tracking any animations for
-    // a node.
-    animation_states.retain(|_, state| !state.is_empty());
-
-    let have_running_animations = animation_states
-        .values()
-        .any(|state| state.needs_animation_ticks());
-    let present = match (had_running_animations, have_running_animations) {
-        (true, false) => AnimationsPresentState::NoAnimationsPresent,
-        (false, true) => AnimationsPresentState::AnimationsPresent,
-        _ => return,
-    };
-    constellation_chan
-        .send(ConstellationMsg::ChangeRunningAnimationsState(
-            pipeline_id,
-            present,
-        ))
-        .unwrap();
-}
-
-/// 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, ElementAnimationSet>,
-    out: &mut Vec<UntrustedNodeAddress>,
-) {
-    // This extends the output vector with an iterator that contains a copy of the node
-    // address for every new animation. The script thread currently stores a rooted node
-    // for every property that is transitioning. The current strategy of repeating the
-    // node address is a holdover from when the code here looked different.
-    out.extend(animation_states.iter().flat_map(|(node, state)| {
-        let mut num_new_animations = state
-            .animations
-            .iter()
-            .filter(|animation| animation.is_new)
-            .count();
-        num_new_animations += state
-            .transitions
-            .iter()
-            .filter(|transition| transition.is_new)
-            .count();
-        std::iter::repeat(node.to_untrusted_node_address()).take(num_new_animations)
-    }));
-}
-
-/// Cancel animations for any nodes which have been removed from the DOM or are display:none.
-/// We detect this by looking for nodes that are used in the flow tree.
-/// TODO(mrobinson): We should look into a way of doing this during flow tree construction.
-/// This also doesn't yet handles nodes that have been reparented.
-pub fn cancel_animations_for_disconnected_nodes(
-    animation_states: &mut FxHashMap<OpaqueNode, ElementAnimationSet>,
-    root_flow: &mut dyn Flow,
-) {
-    // Assume all nodes have been removed until proven otherwise.
-    let mut invalid_nodes: FxHashSet<OpaqueNode> = animation_states.keys().cloned().collect();
-    fn traverse_flow(flow: &mut dyn Flow, invalid_nodes: &mut FxHashSet<OpaqueNode>) {
-        flow.mutate_fragments(&mut |fragment| {
-            invalid_nodes.remove(&fragment.node);
-        });
-        for kid in flow.mut_base().children.iter_mut() {
-            traverse_flow(kid, invalid_nodes)
-        }
-    }
-    traverse_flow(root_flow, &mut invalid_nodes);
-
-    // Cancel animations for any nodes that are no longer in the flow tree.
-    for node in &invalid_nodes {
-        if let Some(state) = animation_states.get_mut(node) {
-            state.cancel_all_animations();
-        }
-    }
-}
-
-fn update_animation_state(
-    script_channel: &IpcSender<ConstellationControlMsg>,
-    animation_state: &mut ElementAnimationSet,
-    pipeline_id: PipelineId,
-    now: f64,
-    node: OpaqueNode,
-) {
-    let send_event = |property_or_animation_name, event_type, elapsed_time| {
-        script_channel
-            .send(ConstellationControlMsg::TransitionOrAnimationEvent(
-                TransitionOrAnimationEvent {
-                    pipeline_id,
-                    event_type,
-                    node: node.to_untrusted_node_address(),
-                    property_or_animation_name,
-                    elapsed_time,
-                },
-            ))
-            .unwrap()
-    };
-
-    handle_canceled_animations(animation_state, now, send_event);
-    finish_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
-/// have ended.
-fn finish_running_animations(
-    animation_state: &mut ElementAnimationSet,
-    now: f64,
-    mut send_event: impl FnMut(String, TransitionOrAnimationEventType, f64),
-) {
-    for animation in animation_state.animations.iter_mut() {
-        if animation.state == AnimationState::Running && animation.has_ended(now) {
-            animation.state = AnimationState::Finished;
-            send_event(
-                animation.name.to_string(),
-                TransitionOrAnimationEventType::AnimationEnd,
-                animation.active_duration(),
-            );
-        }
-    }
-
-    for transition in animation_state.transitions.iter_mut() {
-        if transition.state == AnimationState::Running && transition.has_ended(now) {
-            transition.state = AnimationState::Finished;
-            send_event(
-                transition.property_animation.property_name().into(),
-                TransitionOrAnimationEventType::TransitionEnd,
-                transition.property_animation.duration,
-            );
-        }
-    }
-}
-
-/// Send events for canceled animations. Currently this only handles canceled
-/// transitions, but eventually this should handle canceled CSS animations as
-/// well.
-fn handle_canceled_animations(
-    animation_state: &mut ElementAnimationSet,
-    now: f64,
-    mut send_event: impl FnMut(String, TransitionOrAnimationEventType, f64),
-) {
-    for transition in &animation_state.transitions {
-        if transition.state == AnimationState::Canceled {
-            // TODO(mrobinson): We need to properly compute the elapsed_time here
-            // according to https://drafts.csswg.org/css-transitions/#event-transitionevent
-            send_event(
-                transition.property_animation.property_name().into(),
-                TransitionOrAnimationEventType::TransitionCancel,
-                (now - transition.start_time).max(0.),
-            );
-        }
-    }
-
-    // TODO(mrobinson): We need to send animationcancel events.
-    animation_state.clear_canceled_animations();
-}
-
-fn handle_new_animations(
-    animation_state: &mut ElementAnimationSet,
-    mut send_event: impl FnMut(String, TransitionOrAnimationEventType, f64),
-) {
-    for animation in animation_state.animations.iter_mut() {
-        animation.is_new = false;
-    }
-
-    for transition in animation_state.transitions.iter_mut() {
-        if transition.is_new {
-            // TODO(mrobinson): We need to properly compute the elapsed_time here
-            // according to https://drafts.csswg.org/css-transitions/#event-transitionevent
-            send_event(
-                transition.property_animation.property_name().into(),
-                TransitionOrAnimationEventType::TransitionRun,
-                0.,
-            );
-            transition.is_new = false;
-        }
-    }
-}
diff --git a/components/layout/context.rs b/components/layout/context.rs
index 00e0c5a0cc1..a17bc1db4ee 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -17,7 +17,6 @@ use net_traits::image_cache::{
 use parking_lot::RwLock;
 use script_layout_interface::{PendingImage, PendingImageState};
 use script_traits::Painter;
-use script_traits::UntrustedNodeAddress;
 use servo_atoms::Atom;
 use servo_url::{ImmutableOrigin, ServoUrl};
 use std::cell::{RefCell, RefMut};
@@ -85,9 +84,6 @@ pub struct LayoutContext<'a> {
 
     /// A list of in-progress image loads to be shared with the script thread.
     pub pending_images: Mutex<Vec<PendingImage>>,
-
-    /// A list of nodes that have just initiated a CSS transition or animation.
-    pub newly_animating_nodes: Mutex<Vec<UntrustedNodeAddress>>,
 }
 
 impl<'a> Drop for LayoutContext<'a> {
diff --git a/components/layout/lib.rs b/components/layout/lib.rs
index fdf3971fc3f..98e04f7d93e 100644
--- a/components/layout/lib.rs
+++ b/components/layout/lib.rs
@@ -20,7 +20,6 @@ extern crate serde;
 #[macro_use]
 pub mod layout_debug;
 
-pub mod animation;
 mod block;
 pub mod construct;
 pub mod context;
diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index ff0b11875f7..037fffc34ec 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -29,7 +29,7 @@ use crossbeam_channel::{Receiver, Sender};
 use embedder_traits::resources::{self, Resource};
 use euclid::{default::Size2D as UntypedSize2D, Point2D, Rect, Scale, Size2D};
 use fnv::FnvHashMap;
-use fxhash::FxHashMap;
+use fxhash::{FxHashMap, FxHashSet};
 use gfx::font;
 use gfx::font_cache_thread::FontCacheThread;
 use gfx::font_context;
@@ -37,7 +37,6 @@ use gfx_traits::{node_id_from_scroll_id, Epoch};
 use histogram::Histogram;
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use ipc_channel::router::ROUTER;
-use layout::animation;
 use layout::construct::ConstructionResult;
 use layout::context::malloc_size_of_persistent_local_context;
 use layout::context::LayoutContext;
@@ -191,9 +190,6 @@ pub struct LayoutThread {
     /// The document-specific shared lock used for author-origin stylesheets
     document_shared_lock: Option<SharedRwLock>,
 
-    /// The animation state for all of our nodes.
-    animation_states: ServoArc<RwLock<FxHashMap<OpaqueNode, ElementAnimationSet>>>,
-
     /// A counter for epoch messages
     epoch: Cell<Epoch>,
 
@@ -552,7 +548,6 @@ impl LayoutThread {
             outstanding_web_fonts: Arc::new(AtomicUsize::new(0)),
             root_flow: RefCell::new(None),
             document_shared_lock: None,
-            animation_states: ServoArc::new(RwLock::new(Default::default())),
             // Epoch starts at 1 because of the initial display list for epoch 0 that we send to WR
             epoch: Cell::new(Epoch(1)),
             viewport_size: Size2D::new(Au(0), Au(0)),
@@ -613,6 +608,7 @@ impl LayoutThread {
         snapshot_map: &'a SnapshotMap,
         origin: ImmutableOrigin,
         animation_timeline_value: f64,
+        animation_states: ServoArc<RwLock<FxHashMap<OpaqueNode, ElementAnimationSet>>>,
     ) -> LayoutContext<'a> {
         LayoutContext {
             id: self.id,
@@ -622,7 +618,7 @@ impl LayoutThread {
                 options: GLOBAL_STYLE_DATA.options.clone(),
                 guards,
                 visited_styles_enabled: false,
-                animation_states: self.animation_states.clone(),
+                animation_states,
                 registered_speculative_painters: &self.registered_painters,
                 current_time_for_animations: animation_timeline_value,
                 traversal_flags: TraversalFlags::empty(),
@@ -632,7 +628,6 @@ impl LayoutThread {
             font_cache_thread: Mutex::new(self.font_cache_thread.clone()),
             webrender_image_cache: self.webrender_image_cache.clone(),
             pending_images: Mutex::new(vec![]),
-            newly_animating_nodes: Mutex::new(vec![]),
             registered_painters: &self.registered_painters,
         }
     }
@@ -657,7 +652,6 @@ impl LayoutThread {
             },
             Msg::RegisterPaint(..) => LayoutHangAnnotation::RegisterPaint,
             Msg::SetNavigationStart(..) => LayoutHangAnnotation::SetNavigationStart,
-            Msg::GetRunningAnimations(..) => LayoutHangAnnotation::GetRunningAnimations,
         };
         self.background_hang_monitor
             .as_ref()
@@ -824,15 +818,6 @@ impl LayoutThread {
             Msg::SetNavigationStart(time) => {
                 self.paint_time_metrics.set_navigation_start(time);
             },
-            Msg::GetRunningAnimations(sender) => {
-                let running_animation_count = self
-                    .animation_states
-                    .read()
-                    .values()
-                    .map(|state| state.running_animation_and_transition_count())
-                    .sum();
-                let _ = sender.send(running_animation_count);
-            },
         }
 
         true
@@ -1413,8 +1398,13 @@ impl LayoutThread {
         self.stylist.flush(&guards, Some(element), Some(&map));
 
         // Create a layout context for use throughout the following passes.
-        let mut layout_context =
-            self.build_layout_context(guards.clone(), &map, origin, data.animation_timeline_value);
+        let mut layout_context = self.build_layout_context(
+            guards.clone(),
+            &map,
+            origin,
+            data.animation_timeline_value,
+            data.animations.clone(),
+        );
 
         let pool;
         let (thread_pool, num_threads) = if self.parallel_flag {
@@ -1516,8 +1506,6 @@ impl LayoutThread {
     ) {
         reflow_result.pending_images =
             std::mem::replace(&mut *context.pending_images.lock().unwrap(), vec![]);
-        reflow_result.newly_animating_nodes =
-            std::mem::replace(&mut *context.newly_animating_nodes.lock().unwrap(), vec![]);
 
         let mut root_flow = match self.root_flow.borrow().clone() {
             Some(root_flow) => root_flow,
@@ -1629,6 +1617,33 @@ impl LayoutThread {
         rw_data.scroll_offsets = layout_scroll_states
     }
 
+    /// Cancel animations for any nodes which have been removed from flow tree.
+    /// TODO(mrobinson): We should look into a way of doing this during flow tree construction.
+    /// This also doesn't yet handles nodes that have been reparented.
+    fn cancel_animations_for_nodes_not_in_flow_tree(
+        animation_states: &mut FxHashMap<OpaqueNode, ElementAnimationSet>,
+        root_flow: &mut dyn Flow,
+    ) {
+        // Assume all nodes have been removed until proven otherwise.
+        let mut invalid_nodes: FxHashSet<OpaqueNode> = animation_states.keys().cloned().collect();
+        fn traverse_flow(flow: &mut dyn Flow, invalid_nodes: &mut FxHashSet<OpaqueNode>) {
+            flow.mutate_fragments(&mut |fragment| {
+                invalid_nodes.remove(&fragment.node);
+            });
+            for kid in flow.mut_base().children.iter_mut() {
+                traverse_flow(kid, invalid_nodes)
+            }
+        }
+        traverse_flow(root_flow, &mut invalid_nodes);
+
+        // Cancel animations for any nodes that are no longer in the flow tree.
+        for node in &invalid_nodes {
+            if let Some(state) = animation_states.get_mut(node) {
+                state.cancel_all_animations();
+            }
+        }
+    }
+
     fn perform_post_style_recalc_layout_passes(
         &self,
         root_flow: &mut FlowRef,
@@ -1638,18 +1653,10 @@ impl LayoutThread {
         rw_data: &mut LayoutThreadData,
         context: &mut LayoutContext,
     ) {
-        {
-            let mut newly_animating_nodes = context.newly_animating_nodes.lock().unwrap();
-            animation::do_post_style_animations_update(
-                &self.constellation_chan,
-                &self.script_chan,
-                &mut *(self.animation_states.write()),
-                self.id,
-                context.style_context.current_time_for_animations,
-                &mut newly_animating_nodes,
-                FlowRef::deref_mut(root_flow),
-            );
-        }
+        Self::cancel_animations_for_nodes_not_in_flow_tree(
+            &mut *(context.style_context.animation_states.write()),
+            FlowRef::deref_mut(root_flow),
+        );
 
         profile(
             profile_time::ProfilerCategory::LayoutRestyleDamagePropagation,
diff --git a/components/layout_thread_2020/lib.rs b/components/layout_thread_2020/lib.rs
index 42a5a4d3e00..d97c29459ac 100644
--- a/components/layout_thread_2020/lib.rs
+++ b/components/layout_thread_2020/lib.rs
@@ -615,7 +615,6 @@ impl LayoutThread {
             },
             Msg::RegisterPaint(..) => LayoutHangAnnotation::RegisterPaint,
             Msg::SetNavigationStart(..) => LayoutHangAnnotation::SetNavigationStart,
-            Msg::GetRunningAnimations(..) => LayoutHangAnnotation::GetRunningAnimations,
         };
         self.background_hang_monitor
             .as_ref()
@@ -766,9 +765,6 @@ impl LayoutThread {
             Msg::SetNavigationStart(time) => {
                 self.paint_time_metrics.set_navigation_start(time);
             },
-            Msg::GetRunningAnimations(sender) => {
-                let _ = sender.send(0);
-            },
         }
 
         true
diff --git a/components/msg/constellation_msg.rs b/components/msg/constellation_msg.rs
index c70021421d6..8a599405df8 100644
--- a/components/msg/constellation_msg.rs
+++ b/components/msg/constellation_msg.rs
@@ -541,7 +541,6 @@ pub enum LayoutHangAnnotation {
     UpdateScrollStateFromScript,
     RegisterPaint,
     SetNavigationStart,
-    GetRunningAnimations,
 }
 
 #[derive(Clone, Copy, Debug, Deserialize, Serialize)]
diff --git a/components/script/Cargo.toml b/components/script/Cargo.toml
index f4d8808c869..9f98ff30909 100644
--- a/components/script/Cargo.toml
+++ b/components/script/Cargo.toml
@@ -53,6 +53,7 @@ encoding_rs = "0.8"
 enum-iterator = "0.3"
 euclid = "0.20"
 fnv = "1.0"
+fxhash = "0.2"
 headers = "0.2"
 html5ever = "0.25"
 http = "0.1"
diff --git a/components/script/animation_timeline.rs b/components/script/animation_timeline.rs
index e0ad520db61..54127b94e9e 100644
--- a/components/script/animation_timeline.rs
+++ b/components/script/animation_timeline.rs
@@ -12,7 +12,7 @@ use time;
 /// A `AnimationTimeline` which is used to synchronize animations during the script
 /// event loop.
 #[derive(Clone, Copy, Debug, JSTraceable, MallocSizeOf)]
-pub struct AnimationTimeline {
+pub(crate) struct AnimationTimeline {
     current_value: f64,
 }
 
diff --git a/components/script/animations.rs b/components/script/animations.rs
new file mode 100644
index 00000000000..a666ac3ed84
--- /dev/null
+++ b/components/script/animations.rs
@@ -0,0 +1,293 @@
+/* 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/. */
+
+#![deny(missing_docs)]
+
+//! The set of animations for a document.
+
+use crate::dom::window::Window;
+use fxhash::FxHashMap;
+use libc::c_void;
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
+use msg::constellation_msg::PipelineId;
+use parking_lot::RwLock;
+use script_traits::{AnimationState as AnimationsPresentState, ScriptMsg, UntrustedNodeAddress};
+use servo_arc::Arc;
+use style::animation::{AnimationState, ElementAnimationSet};
+use style::dom::OpaqueNode;
+
+/// The set of animations for a document.
+///
+/// Make sure to update the MallocSizeOf implementation when changing the
+/// contents of this struct.
+#[derive(Clone, Debug, Default, JSTraceable)]
+pub(crate) struct Animations {
+    pub sets: Arc<RwLock<FxHashMap<OpaqueNode, ElementAnimationSet>>>,
+    have_running_animations: bool,
+}
+
+impl Animations {
+    pub(crate) fn new() -> Self {
+        Animations {
+            sets: Default::default(),
+            have_running_animations: false,
+        }
+    }
+
+    /// Processes any new animations that were discovered after reflow. Collect messages
+    /// that trigger events for any animations that changed state.
+    /// TODO(mrobinson): The specification dictates that this should happen before reflow.
+    pub(crate) fn do_post_reflow_update(&mut self, window: &Window, now: f64) -> AnimationsUpdate {
+        let mut update = AnimationsUpdate::new(window.pipeline_id());
+
+        {
+            let mut sets = self.sets.write();
+            update.collect_newly_animating_nodes(&sets);
+
+            for set in sets.values_mut() {
+                Self::handle_canceled_animations(set, now, &mut update);
+                Self::finish_running_animations(set, now, &mut update);
+                Self::handle_new_animations(set, &mut update);
+            }
+
+            // Remove empty states from our collection of states in order to free
+            // up space as soon as we are no longer tracking any animations for
+            // a node.
+            sets.retain(|_, state| !state.is_empty());
+        }
+
+        self.update_running_animations_presence(window);
+
+        update
+    }
+
+    pub(crate) fn running_animation_count(&self) -> usize {
+        self.sets
+            .read()
+            .values()
+            .map(|state| state.running_animation_and_transition_count())
+            .sum()
+    }
+
+    fn update_running_animations_presence(&mut self, window: &Window) {
+        let have_running_animations = self
+            .sets
+            .read()
+            .values()
+            .any(|state| state.needs_animation_ticks());
+        if have_running_animations == self.have_running_animations {
+            return;
+        }
+
+        self.have_running_animations = have_running_animations;
+        let state = match have_running_animations {
+            true => AnimationsPresentState::AnimationsPresent,
+            false => AnimationsPresentState::NoAnimationsPresent,
+        };
+
+        window.send_to_constellation(ScriptMsg::ChangeRunningAnimationsState(state));
+    }
+
+    /// Walk through the list of running animations and remove all of the ones that
+    /// have ended.
+    fn finish_running_animations(
+        set: &mut ElementAnimationSet,
+        now: f64,
+        update: &mut AnimationsUpdate,
+    ) {
+        for animation in set.animations.iter_mut() {
+            if animation.state == AnimationState::Running && animation.has_ended(now) {
+                animation.state = AnimationState::Finished;
+                update.add_event(
+                    animation.node,
+                    animation.name.to_string(),
+                    TransitionOrAnimationEventType::AnimationEnd,
+                    animation.active_duration(),
+                );
+            }
+        }
+
+        for transition in set.transitions.iter_mut() {
+            if transition.state == AnimationState::Running && transition.has_ended(now) {
+                transition.state = AnimationState::Finished;
+                update.add_event(
+                    transition.node,
+                    transition.property_animation.property_name().into(),
+                    TransitionOrAnimationEventType::TransitionEnd,
+                    transition.property_animation.duration,
+                );
+            }
+        }
+    }
+
+    /// Send events for canceled animations. Currently this only handles canceled
+    /// transitions, but eventually this should handle canceled CSS animations as
+    /// well.
+    fn handle_canceled_animations(
+        set: &mut ElementAnimationSet,
+        now: f64,
+        update: &mut AnimationsUpdate,
+    ) {
+        for transition in &set.transitions {
+            if transition.state == AnimationState::Canceled {
+                // TODO(mrobinson): We need to properly compute the elapsed_time here
+                // according to https://drafts.csswg.org/css-transitions/#event-transitionevent
+                update.add_event(
+                    transition.node,
+                    transition.property_animation.property_name().into(),
+                    TransitionOrAnimationEventType::TransitionCancel,
+                    (now - transition.start_time).max(0.),
+                );
+            }
+        }
+
+        // TODO(mrobinson): We need to send animationcancel events.
+        set.clear_canceled_animations();
+    }
+
+    fn handle_new_animations(set: &mut ElementAnimationSet, update: &mut AnimationsUpdate) {
+        for animation in set.animations.iter_mut() {
+            animation.is_new = false;
+        }
+
+        for transition in set.transitions.iter_mut() {
+            if transition.is_new {
+                // TODO(mrobinson): We need to properly compute the elapsed_time here
+                // according to https://drafts.csswg.org/css-transitions/#event-transitionevent
+                update.add_event(
+                    transition.node,
+                    transition.property_animation.property_name().into(),
+                    TransitionOrAnimationEventType::TransitionRun,
+                    0.,
+                );
+                transition.is_new = false;
+            }
+        }
+    }
+}
+
+impl MallocSizeOf for Animations {
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        self.sets.read().size_of(ops) + self.have_running_animations.size_of(ops)
+    }
+}
+
+pub(crate) struct AnimationsUpdate {
+    pub pipeline_id: PipelineId,
+    pub events: Vec<TransitionOrAnimationEvent>,
+    pub newly_animating_nodes: Vec<UntrustedNodeAddress>,
+}
+
+impl AnimationsUpdate {
+    fn new(pipeline_id: PipelineId) -> Self {
+        AnimationsUpdate {
+            pipeline_id,
+            events: Default::default(),
+            newly_animating_nodes: Default::default(),
+        }
+    }
+
+    fn add_event(
+        &mut self,
+        node: OpaqueNode,
+        property_or_animation_name: String,
+        event_type: TransitionOrAnimationEventType,
+        elapsed_time: f64,
+    ) {
+        let node = UntrustedNodeAddress(node.0 as *const c_void);
+        self.events.push(TransitionOrAnimationEvent {
+            pipeline_id: self.pipeline_id,
+            event_type,
+            node,
+            property_or_animation_name,
+            elapsed_time,
+        });
+    }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        self.events.is_empty() && self.newly_animating_nodes.is_empty()
+    }
+
+    /// 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.
+    /// TODO(mrobinson): Look into handling the rooting inside this class.
+    fn collect_newly_animating_nodes(
+        &mut self,
+        animation_states: &FxHashMap<OpaqueNode, ElementAnimationSet>,
+    ) {
+        // This extends the output vector with an iterator that contains a copy of the node
+        // address for every new animation. The script thread currently stores a rooted node
+        // for every property that is transitioning. The current strategy of repeating the
+        // node address is a holdover from when the code here looked different.
+        self.newly_animating_nodes
+            .extend(animation_states.iter().flat_map(|(node, state)| {
+                let mut num_new_animations = state
+                    .animations
+                    .iter()
+                    .filter(|animation| animation.is_new)
+                    .count();
+                num_new_animations += state
+                    .transitions
+                    .iter()
+                    .filter(|transition| transition.is_new)
+                    .count();
+
+                let node = UntrustedNodeAddress(node.0 as *const c_void);
+                std::iter::repeat(node).take(num_new_animations)
+            }));
+    }
+}
+
+/// The type of transition event to trigger. These are defined by
+/// CSS Transitions § 6.1 and CSS Animations § 4.2
+#[derive(Clone, Debug, Deserialize, JSTraceable, Serialize)]
+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 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 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,
+        }
+    }
+}
+
+#[derive(Deserialize, JSTraceable, Serialize)]
+/// A transition or animation event.
+pub struct TransitionOrAnimationEvent {
+    /// The pipeline id of the layout task that sent this message.
+    pub pipeline_id: PipelineId,
+    /// The type of transition event this should trigger.
+    pub event_type: TransitionOrAnimationEventType,
+    /// The address of the node which owns this transition.
+    pub node: UntrustedNodeAddress,
+    /// 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).
+    pub property_or_animation_name: String,
+    /// The elapsed time property to send with this transition event.
+    pub elapsed_time: f64,
+}
diff --git a/components/script/dom/bindings/trace.rs b/components/script/dom/bindings/trace.rs
index c2543cc984b..26db47233f9 100644
--- a/components/script/dom/bindings/trace.rs
+++ b/components/script/dom/bindings/trace.rs
@@ -93,6 +93,7 @@ use net_traits::response::HttpsState;
 use net_traits::response::{Response, ResponseBody};
 use net_traits::storage_thread::StorageType;
 use net_traits::{Metadata, NetworkError, ReferrerPolicy, ResourceFetchTiming, ResourceThreads};
+use parking_lot::RwLock;
 use profile_traits::mem::ProfilerChan as MemProfilerChan;
 use profile_traits::time::ProfilerChan as TimeProfilerChan;
 use script_layout_interface::message::PendingRestyle;
@@ -100,9 +101,11 @@ use script_layout_interface::rpc::LayoutRPC;
 use script_layout_interface::StyleAndOpaqueLayoutData;
 use script_traits::serializable::BlobImpl;
 use script_traits::transferable::MessagePortImpl;
-use script_traits::{DocumentActivity, DrawAPaintImageResult};
-use script_traits::{MediaSessionActionType, ScriptToConstellationChan, TimerEventId, TimerSource};
-use script_traits::{UntrustedNodeAddress, WebrenderIpcSender, WindowSizeData, WindowSizeType};
+use script_traits::{
+    DocumentActivity, DrawAPaintImageResult, MediaSessionActionType, ScriptToConstellationChan,
+    TimerEventId, TimerSource, UntrustedNodeAddress, WebrenderIpcSender, WindowSizeData,
+    WindowSizeType,
+};
 use selectors::matching::ElementSelectorFlags;
 use serde::{Deserialize, Serialize};
 use servo_arc::Arc as ServoArc;
@@ -131,6 +134,7 @@ use std::rc::Rc;
 use std::sync::atomic::{AtomicBool, AtomicUsize};
 use std::sync::{Arc, Mutex};
 use std::time::{Instant, SystemTime};
+use style::animation::ElementAnimationSet;
 use style::attr::{AttrIdentifier, AttrValue, LengthOrPercentageOrAuto};
 use style::author_styles::AuthorStyles;
 use style::context::QuirksMode;
@@ -172,7 +176,6 @@ unsafe_no_jsmanaged_fields!(Box<dyn TaskBox>, Box<dyn EventLoopWaker>);
 
 unsafe_no_jsmanaged_fields!(MessagePortImpl);
 unsafe_no_jsmanaged_fields!(MessagePortId);
-unsafe_no_jsmanaged_fields!(RefCell<Option<MessagePortId>>);
 unsafe_no_jsmanaged_fields!(MessagePortRouterId);
 
 unsafe_no_jsmanaged_fields!(BroadcastChannelRouterId);
@@ -184,8 +187,7 @@ unsafe_no_jsmanaged_fields!(CSSError);
 
 unsafe_no_jsmanaged_fields!(&'static Encoding);
 
-unsafe_no_jsmanaged_fields!(RefCell<Decoder>);
-unsafe_no_jsmanaged_fields!(RefCell<Vec<u8>>);
+unsafe_no_jsmanaged_fields!(Decoder);
 
 unsafe_no_jsmanaged_fields!(Reflector);
 
@@ -252,6 +254,12 @@ unsafe impl<T: JSTraceable> JSTraceable for ServoArc<T> {
     }
 }
 
+unsafe impl<T: JSTraceable> JSTraceable for RwLock<T> {
+    unsafe fn trace(&self, trc: *mut JSTracer) {
+        self.read().trace(trc)
+    }
+}
+
 unsafe impl<T: JSTraceable + ?Sized> JSTraceable for Box<T> {
     unsafe fn trace(&self, trc: *mut JSTracer) {
         (**self).trace(trc)
@@ -284,6 +292,12 @@ unsafe impl<T: JSTraceable> JSTraceable for DomRefCell<T> {
     }
 }
 
+unsafe impl<T: JSTraceable> JSTraceable for RefCell<T> {
+    unsafe fn trace(&self, trc: *mut JSTracer) {
+        (*self).borrow().trace(trc)
+    }
+}
+
 unsafe impl JSTraceable for Heap<*mut JSObject> {
     unsafe fn trace(&self, trc: *mut JSTracer) {
         if self.get().is_null() {
@@ -530,8 +544,7 @@ unsafe_no_jsmanaged_fields!(WebGLTextureId);
 unsafe_no_jsmanaged_fields!(WebGLVertexArrayId);
 unsafe_no_jsmanaged_fields!(WebGLVersion);
 unsafe_no_jsmanaged_fields!(WebGLSLVersion);
-unsafe_no_jsmanaged_fields!(RefCell<Option<WebGPU>>);
-unsafe_no_jsmanaged_fields!(RefCell<Identities>);
+unsafe_no_jsmanaged_fields!(Identities);
 unsafe_no_jsmanaged_fields!(WebGPU);
 unsafe_no_jsmanaged_fields!(WebGPUAdapter);
 unsafe_no_jsmanaged_fields!(WebGPUBuffer);
@@ -544,7 +557,7 @@ unsafe_no_jsmanaged_fields!(WebGPUShaderModule);
 unsafe_no_jsmanaged_fields!(WebGPUCommandBuffer);
 unsafe_no_jsmanaged_fields!(WebGPUCommandEncoder);
 unsafe_no_jsmanaged_fields!(WebGPUDevice);
-unsafe_no_jsmanaged_fields!(RefCell<Option<RawPass>>);
+unsafe_no_jsmanaged_fields!(Option<RawPass>);
 unsafe_no_jsmanaged_fields!(GPUBufferState);
 unsafe_no_jsmanaged_fields!(WebXRSwapChainId);
 unsafe_no_jsmanaged_fields!(MediaList);
@@ -586,6 +599,7 @@ unsafe_no_jsmanaged_fields!(MediaSessionActionType);
 unsafe_no_jsmanaged_fields!(MediaMetadata);
 unsafe_no_jsmanaged_fields!(WebrenderIpcSender);
 unsafe_no_jsmanaged_fields!(StreamConsumer);
+unsafe_no_jsmanaged_fields!(ElementAnimationSet);
 
 unsafe impl<'a> JSTraceable for &'a str {
     #[inline]
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index 5e40f433da5..86c5b1caaa1 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -3,6 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use crate::animation_timeline::AnimationTimeline;
+use crate::animations::{Animations, AnimationsUpdate};
 use crate::document_loader::{DocumentLoader, LoadType};
 use crate::dom::attr::Attr;
 use crate::dom::beforeunloadevent::BeforeUnloadEvent;
@@ -384,6 +385,8 @@ pub struct Document {
     /// A timeline for animations which is used for synchronizing animations.
     /// https://drafts.csswg.org/web-animations/#timeline
     animation_timeline: DomRefCell<AnimationTimeline>,
+    /// Animations for this Document
+    animations: DomRefCell<Animations>,
 }
 
 #[derive(JSTraceable, MallocSizeOf)]
@@ -2913,6 +2916,7 @@ impl Document {
             } else {
                 DomRefCell::new(AnimationTimeline::new())
             },
+            animations: DomRefCell::new(Animations::new()),
         }
     }
 
@@ -3615,17 +3619,27 @@ impl Document {
             .collect()
     }
 
-    pub fn advance_animation_timeline_for_testing(&self, delta: f64) {
+    pub(crate) fn advance_animation_timeline_for_testing(&self, delta: f64) {
         self.animation_timeline.borrow_mut().advance_specific(delta);
     }
 
-    pub fn update_animation_timeline(&self) {
+    pub(crate) fn update_animation_timeline(&self) {
         self.animation_timeline.borrow_mut().update();
     }
 
-    pub fn current_animation_timeline_value(&self) -> f64 {
+    pub(crate) fn current_animation_timeline_value(&self) -> f64 {
         self.animation_timeline.borrow().current_value()
     }
+
+    pub(crate) fn animations(&self) -> Ref<Animations> {
+        self.animations.borrow()
+    }
+
+    pub(crate) fn update_animations(&self) -> AnimationsUpdate {
+        self.animations
+            .borrow_mut()
+            .do_post_reflow_update(&self.window, self.current_animation_timeline_value())
+    }
 }
 
 impl Element {
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 414ef0f2e45..cbf19039605 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -81,7 +81,7 @@ use dom_struct::dom_struct;
 use embedder_traits::{EmbedderMsg, EventLoopWaker, PromptDefinition, PromptOrigin, PromptResult};
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect};
 use euclid::{Point2D, Rect, Scale, Size2D, Vector2D};
-use ipc_channel::ipc::{channel, IpcSender};
+use ipc_channel::ipc::IpcSender;
 use ipc_channel::router::ROUTER;
 use js::jsapi::Heap;
 use js::jsapi::JSAutoRealm;
@@ -1305,9 +1305,9 @@ impl WindowMethods for Window {
     }
 
     fn RunningAnimationCount(&self) -> u32 {
-        let (sender, receiver) = channel().unwrap();
-        let _ = self.layout_chan.send(Msg::GetRunningAnimations(sender));
-        receiver.recv().unwrap_or(0) as u32
+        self.document
+            .get()
+            .map_or(0, |d| d.animations().running_animation_count() as u32)
     }
 
     // https://html.spec.whatwg.org/multipage/#dom-name
@@ -1643,6 +1643,7 @@ impl Window {
             dom_count: document.dom_count(),
             pending_restyles: document.drain_pending_restyles(),
             animation_timeline_value: document.current_animation_timeline_value(),
+            animations: document.animations().sets.clone(),
         };
 
         self.layout_chan
@@ -1706,8 +1707,9 @@ impl Window {
             }
         }
 
+        let update = document.update_animations();
         unsafe {
-            ScriptThread::note_newly_animating_nodes(pipeline_id, complete.newly_animating_nodes);
+            ScriptThread::process_animations_update(update);
         }
 
         true
diff --git a/components/script/lib.rs b/components/script/lib.rs
index cd73045456a..886a76e9c95 100644
--- a/components/script/lib.rs
+++ b/components/script/lib.rs
@@ -48,6 +48,7 @@ extern crate servo_atoms;
 extern crate style;
 
 mod animation_timeline;
+mod animations;
 #[warn(deprecated)]
 #[macro_use]
 mod task;
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 008274ded47..9754aa65ede 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -17,6 +17,9 @@
 //! a page runs its course and the script thread returns to processing events in the main event
 //! loop.
 
+use crate::animations::{
+    AnimationsUpdate, TransitionOrAnimationEvent, TransitionOrAnimationEventType,
+};
 use crate::devtools;
 use crate::document_loader::DocumentLoader;
 use crate::dom::animationevent::AnimationEvent;
@@ -141,8 +144,8 @@ use script_traits::{
     LayoutMsg, LoadData, LoadOrigin, MediaSessionActionType, MouseButton, MouseEventType,
     NewLayoutInfo, Painter, ProgressiveWebMetricType, ScriptMsg, ScriptThreadFactory,
     ScriptToConstellationChan, StructuredSerializedData, TimerSchedulerMsg, TouchEventType,
-    TouchId, TransitionOrAnimationEvent, TransitionOrAnimationEventType, UntrustedNodeAddress,
-    UpdatePipelineIdReason, WebrenderIpcSender, WheelDelta, WindowSizeData, WindowSizeType,
+    TouchId, UntrustedNodeAddress, UpdatePipelineIdReason, WebrenderIpcSender, WheelDelta,
+    WindowSizeData, WindowSizeType,
 };
 use servo_atoms::Atom;
 use servo_config::opts;
@@ -510,10 +513,8 @@ impl<'a> Iterator for DocumentsIter<'a> {
 // thread during parsing. For this reason, we don't trace the
 // incomplete parser contexts during GC.
 type IncompleteParserContexts = Vec<(PipelineId, ParserContext)>;
-unsafe_no_jsmanaged_fields!(RefCell<IncompleteParserContexts>);
 
 unsafe_no_jsmanaged_fields!(TaskQueue<MainThreadScriptMsg>);
-
 unsafe_no_jsmanaged_fields!(dyn BackgroundHangMonitorRegister);
 unsafe_no_jsmanaged_fields!(dyn BackgroundHangMonitor);
 
@@ -644,6 +645,9 @@ pub struct ScriptThread {
     /// of the transition.
     animating_nodes: DomRefCell<HashMap<PipelineId, Vec<Dom<Node>>>>,
 
+    /// Animations events that are pending to be sent.
+    animation_events: RefCell<Vec<TransitionOrAnimationEvent>>,
+
     /// <https://html.spec.whatwg.org/multipage/#custom-element-reactions-stack>
     custom_element_reaction_stack: CustomElementReactionStack,
 
@@ -826,20 +830,35 @@ impl ScriptThread {
         })
     }
 
-    pub unsafe fn note_newly_animating_nodes(
-        pipeline_id: PipelineId,
-        nodes: Vec<UntrustedNodeAddress>,
-    ) {
+    /// Consume the list of pointer addresses corresponding to DOM nodes that are animating
+    /// and root them in a per-pipeline list of nodes.
+    ///
+    /// Unsafety: any pointer to invalid memory (ie. a GCed node) will trigger a crash.
+    /// TODO: ensure caller uses rooted nodes instead of unsafe node addresses.
+    pub(crate) unsafe fn process_animations_update(mut update: AnimationsUpdate) {
+        if update.is_empty() {
+            return;
+        }
+
         SCRIPT_THREAD_ROOT.with(|root| {
             let script_thread = &*root.get().unwrap();
+
+            if !update.events.is_empty() {
+                script_thread
+                    .animation_events
+                    .borrow_mut()
+                    .append(&mut update.events);
+            }
+
             let js_runtime = script_thread.js_runtime.rt();
-            let new_nodes = nodes
+            let new_nodes = update
+                .newly_animating_nodes
                 .into_iter()
                 .map(|n| Dom::from_ref(&*from_untrusted_node_address(js_runtime, n)));
             script_thread
                 .animating_nodes
                 .borrow_mut()
-                .entry(pipeline_id)
+                .entry(update.pipeline_id)
                 .or_insert_with(Vec::new)
                 .extend(new_nodes);
         })
@@ -1354,6 +1373,7 @@ impl ScriptThread {
             docs_with_no_blocking_loads: Default::default(),
 
             animating_nodes: Default::default(),
+            animation_events: Default::default(),
 
             custom_element_reaction_stack: CustomElementReactionStack::new(),
 
@@ -1620,9 +1640,8 @@ impl ScriptThread {
     }
 
     // Perform step 11.10 from https://html.spec.whatwg.org/multipage/#event-loops.
-    // TODO(mrobinson): This should also update the current animations and send events
-    // to conform to the HTML specification. This might mean having events for rooting
-    // DOM nodes and ultimately all animations living in script.
+    // TODO(mrobinson): This should also update the current animations to conform to
+    // the HTML specification.
     fn update_animations_and_send_events(&self) {
         for (_, document) in self.documents.borrow().iter() {
             // Only update the time if it isn't being managed by a test.
@@ -1630,6 +1649,13 @@ impl ScriptThread {
                 document.update_animation_timeline();
             }
         }
+
+        // We remove the events because handling these events might trigger
+        // a reflow which might want to add more events to the queue.
+        let events = self.animation_events.replace(Vec::new());
+        for event in events.into_iter() {
+            self.handle_transition_or_animation_event(&event);
+        }
     }
 
     fn categorize_msg(&self, msg: &MixedMessage) -> ScriptThreadEventCategory {
@@ -1720,7 +1746,6 @@ impl ScriptThread {
                 FocusIFrame(id, ..) => Some(id),
                 WebDriverScriptCommand(id, ..) => Some(id),
                 TickAllAnimations(id, ..) => Some(id),
-                TransitionOrAnimationEvent { .. } => None,
                 WebFontLoaded(id) => Some(id),
                 DispatchIFrameLoadEvent {
                     target: _,
@@ -1921,9 +1946,6 @@ impl ScriptThread {
             ConstellationControlMsg::TickAllAnimations(pipeline_id, tick_type) => {
                 self.handle_tick_all_animations(pipeline_id, tick_type)
             },
-            ConstellationControlMsg::TransitionOrAnimationEvent(ref event) => {
-                self.handle_transition_or_animation_event(event);
-            },
             ConstellationControlMsg::WebFontLoaded(pipeline_id) => {
                 self.handle_web_font_loaded(pipeline_id)
             },
diff --git a/components/script_layout_interface/Cargo.toml b/components/script_layout_interface/Cargo.toml
index 8f759970c62..80724ac1295 100644
--- a/components/script_layout_interface/Cargo.toml
+++ b/components/script_layout_interface/Cargo.toml
@@ -16,6 +16,7 @@ atomic_refcell = "0.1"
 canvas_traits = {path = "../canvas_traits"}
 crossbeam-channel = "0.4"
 euclid = "0.20"
+fxhash = "0.2"
 gfx_traits = {path = "../gfx_traits"}
 html5ever = "0.25"
 ipc-channel = "0.14"
@@ -25,6 +26,7 @@ malloc_size_of_derive = "0.1"
 metrics = {path = "../metrics"}
 msg = {path = "../msg"}
 net_traits = {path = "../net_traits"}
+parking_lot = "0.9"
 profile_traits = {path = "../profile_traits"}
 range = {path = "../range"}
 script_traits = {path = "../script_traits"}
diff --git a/components/script_layout_interface/message.rs b/components/script_layout_interface/message.rs
index ac5c0c59ba0..1dcbfe7eb33 100644
--- a/components/script_layout_interface/message.rs
+++ b/components/script_layout_interface/message.rs
@@ -7,20 +7,25 @@ use crate::{PendingImage, TrustedNodeAddress};
 use app_units::Au;
 use crossbeam_channel::{Receiver, Sender};
 use euclid::default::{Point2D, Rect};
+use fxhash::FxHashMap;
 use gfx_traits::Epoch;
 use ipc_channel::ipc::{IpcReceiver, IpcSender};
 use metrics::PaintTimeMetrics;
 use msg::constellation_msg::{BackgroundHangMonitorRegister, BrowsingContextId, PipelineId};
 use net_traits::image_cache::ImageCache;
+use parking_lot::RwLock;
 use profile_traits::mem::ReportsChan;
 use script_traits::Painter;
-use script_traits::{ConstellationControlMsg, LayoutControlMsg, LayoutMsg as ConstellationMsg};
-use script_traits::{ScrollState, UntrustedNodeAddress, WindowSizeData};
+use script_traits::{
+    ConstellationControlMsg, LayoutControlMsg, LayoutMsg as ConstellationMsg, ScrollState,
+    WindowSizeData,
+};
 use servo_arc::Arc as ServoArc;
 use servo_atoms::Atom;
 use servo_url::{ImmutableOrigin, ServoUrl};
 use std::sync::atomic::AtomicBool;
 use std::sync::Arc;
+use style::animation::ElementAnimationSet;
 use style::context::QuirksMode;
 use style::dom::OpaqueNode;
 use style::invalidation::element::restyle_hints::RestyleHint;
@@ -87,9 +92,6 @@ pub enum Msg {
 
     /// Send to layout the precise time when the navigation started.
     SetNavigationStart(u64),
-
-    /// Request the current number of animations that are running.
-    GetRunningAnimations(IpcSender<usize>),
 }
 
 #[derive(Debug, PartialEq)]
@@ -183,8 +185,6 @@ pub struct Reflow {
 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_animating_nodes: Vec<UntrustedNodeAddress>,
 }
 
 /// Information needed for a script-initiated reflow.
@@ -209,6 +209,8 @@ pub struct ScriptReflow {
     pub pending_restyles: Vec<(TrustedNodeAddress, PendingRestyle)>,
     /// The current animation timeline value.
     pub animation_timeline_value: f64,
+    /// The set of animations for this document.
+    pub animations: ServoArc<RwLock<FxHashMap<OpaqueNode, ElementAnimationSet>>>,
 }
 
 pub struct LayoutThreadInit {
diff --git a/components/script_traits/lib.rs b/components/script_traits/lib.rs
index aeac1ff6087..fc3b19d2b97 100644
--- a/components/script_traits/lib.rs
+++ b/components/script_traits/lib.rs
@@ -282,58 +282,6 @@ pub enum UpdatePipelineIdReason {
     Traversal,
 }
 
-/// 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 TransitionOrAnimationEventType {
-    /// "The transitionrun event occurs when a transition is created (i.e., when it
-    /// is added to the set of running transitions)."
-    TransitionRun,
-    /// "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 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,
-        }
-    }
-}
-
-#[derive(Deserialize, Serialize)]
-/// A transition or animation event.
-pub struct TransitionOrAnimationEvent {
-    /// The pipeline id of the layout task that sent this message.
-    pub pipeline_id: PipelineId,
-    /// The type of transition event this should trigger.
-    pub event_type: TransitionOrAnimationEventType,
-    /// The address of the node which owns this transition.
-    pub node: UntrustedNodeAddress,
-    /// 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).
-    pub property_or_animation_name: String,
-    /// The elapsed time property to send with this transition event.
-    pub elapsed_time: f64,
-}
-
 /// Messages sent from the constellation or layout to the script thread.
 #[derive(Deserialize, Serialize)]
 pub enum ConstellationControlMsg {
@@ -420,8 +368,6 @@ pub enum ConstellationControlMsg {
     WebDriverScriptCommand(PipelineId, WebDriverScriptCommand),
     /// Notifies script thread that all animations are done
     TickAllAnimations(PipelineId, AnimationTickType),
-    /// Notifies the script thread that a transition or animation related event should be sent.
-    TransitionOrAnimationEvent(TransitionOrAnimationEvent),
     /// Notifies the script thread that a new Web font has been loaded, and thus the page should be
     /// reflowed.
     WebFontLoaded(PipelineId),
@@ -481,7 +427,6 @@ impl fmt::Debug for ConstellationControlMsg {
             FocusIFrame(..) => "FocusIFrame",
             WebDriverScriptCommand(..) => "WebDriverScriptCommand",
             TickAllAnimations(..) => "TickAllAnimations",
-            TransitionOrAnimationEvent { .. } => "TransitionOrAnimationEvent",
             WebFontLoaded(..) => "WebFontLoaded",
             DispatchIFrameLoadEvent { .. } => "DispatchIFrameLoadEvent",
             DispatchStorageEvent(..) => "DispatchStorageEvent",
diff --git a/components/script_traits/script_msg.rs b/components/script_traits/script_msg.rs
index 245a02ec6d2..b5687d6217e 100644
--- a/components/script_traits/script_msg.rs
+++ b/components/script_traits/script_msg.rs
@@ -61,8 +61,6 @@ pub struct IFrameSizeMsg {
 /// Messages from the layout to the constellation.
 #[derive(Deserialize, Serialize)]
 pub enum LayoutMsg {
-    /// Indicates whether this pipeline is currently running animations.
-    ChangeRunningAnimationsState(PipelineId, AnimationState),
     /// Inform the constellation of the size of the iframe's viewport.
     IFrameSizes(Vec<IFrameSizeMsg>),
     /// Requests that the constellation inform the compositor that it needs to record
@@ -76,7 +74,6 @@ impl fmt::Debug for LayoutMsg {
     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
         use self::LayoutMsg::*;
         let variant = match *self {
-            ChangeRunningAnimationsState(..) => "ChangeRunningAnimationsState",
             IFrameSizes(..) => "IFrameSizes",
             PendingPaintMetric(..) => "PendingPaintMetric",
             ViewportConstrained(..) => "ViewportConstrained",
diff --git a/components/style/animation.rs b/components/style/animation.rs
index e928d81de88..f76759abdfb 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -27,7 +27,7 @@ use servo_arc::Arc;
 use std::fmt;
 
 /// Represents an animation for a given property.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, MallocSizeOf)]
 pub struct PropertyAnimation {
     /// An `AnimatedProperty` that this `PropertyAnimation` corresponds to.
     property: AnimatedProperty,
@@ -136,7 +136,7 @@ impl PropertyAnimation {
 }
 
 /// This structure represents the state of an animation.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
 pub enum AnimationState {
     /// This animation is paused. The inner field is the percentage of progress
     /// when it was paused, from 0 to 1.
@@ -153,7 +153,7 @@ pub enum AnimationState {
 ///
 /// If the iteration count is infinite, there's no other state, otherwise we
 /// have to keep track the current iteration and the max iteration count.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, MallocSizeOf)]
 pub enum KeyframesIterationState {
     /// Infinite iterations, so no need to track a state.
     Infinite,
@@ -164,7 +164,7 @@ pub enum KeyframesIterationState {
 }
 
 /// A CSS Animation
-#[derive(Clone)]
+#[derive(Clone, MallocSizeOf)]
 pub struct Animation {
     /// The node associated with this animation.
     pub node: OpaqueNode,
@@ -198,6 +198,7 @@ pub struct Animation {
 
     /// The original cascade style, needed to compute the generated keyframes of
     /// the animation.
+    #[ignore_malloc_size_of = "ComputedValues"]
     pub cascade_style: Arc<ComputedValues>,
 
     /// Whether or not this animation is new and or has already been tracked
@@ -542,7 +543,7 @@ impl fmt::Debug for Animation {
 }
 
 /// A CSS Transition
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, MallocSizeOf)]
 pub struct Transition {
     /// The node associated with this animation.
     pub node: OpaqueNode,
@@ -597,7 +598,7 @@ impl Transition {
 }
 
 /// Holds the animation state for a particular element.
-#[derive(Debug, Default)]
+#[derive(Debug, Default, MallocSizeOf)]
 pub struct ElementAnimationSet {
     /// The animations for this element.
     pub animations: Vec<Animation>,