diff --git a/components/atoms/static_atoms.txt b/components/atoms/static_atoms.txt
index ebf4a2c27bee..d4f2f38da7e9 100644
--- a/components/atoms/static_atoms.txt
+++ b/components/atoms/static_atoms.txt
@@ -2,6 +2,7 @@ DOMContentLoaded
 abort
 activate
 addtrack
+animationcancel
 animationend
 animationiteration
 animationstart
@@ -132,6 +133,7 @@ track
 transitioncancel
 transitionend
 transitionrun
+transitionstart
 unhandledrejection
 unload
 url
diff --git a/components/script/animations.rs b/components/script/animations.rs
index f9de0fc0a321..d0bfcef6f791 100644
--- a/components/script/animations.rs
+++ b/components/script/animations.rs
@@ -78,12 +78,15 @@ impl Animations {
         let mut sets = self.sets.write();
 
         for set in sets.values_mut() {
+            self.start_pending_animations(set, now, pipeline_id);
+
             // When necessary, iterate our running animations to the next iteration.
             for animation in set.animations.iter_mut() {
                 if animation.iterate_if_necessary(now) {
                     self.add_animation_event(
                         animation,
                         TransitionOrAnimationEventType::AnimationIteration,
+                        now,
                         pipeline_id,
                     );
                 }
@@ -97,7 +100,6 @@ impl Animations {
 
     /// 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(&self, window: &Window, now: f64) {
         let pipeline_id = window.pipeline_id();
         let mut sets = self.sets.write();
@@ -140,6 +142,39 @@ impl Animations {
             .sum()
     }
 
+    /// Walk through the list of pending animations and start all of the ones that
+    /// have left the delay phase.
+    fn start_pending_animations(
+        &self,
+        set: &mut ElementAnimationSet,
+        now: f64,
+        pipeline_id: PipelineId,
+    ) {
+        for animation in set.animations.iter_mut() {
+            if animation.state == AnimationState::Pending && animation.started_at <= now {
+                animation.state = AnimationState::Running;
+                self.add_animation_event(
+                    animation,
+                    TransitionOrAnimationEventType::AnimationStart,
+                    now,
+                    pipeline_id,
+                );
+            }
+        }
+
+        for transition in set.transitions.iter_mut() {
+            if transition.state == AnimationState::Pending && transition.start_time <= now {
+                transition.state = AnimationState::Running;
+                self.add_transition_event(
+                    transition,
+                    TransitionOrAnimationEventType::TransitionStart,
+                    now,
+                    pipeline_id,
+                );
+            }
+        }
+    }
+
     /// Walk through the list of running animations and remove all of the ones that
     /// have ended.
     fn finish_running_animations(
@@ -154,6 +189,7 @@ impl Animations {
                 self.add_animation_event(
                     animation,
                     TransitionOrAnimationEventType::AnimationEnd,
+                    now,
                     pipeline_id,
                 );
             }
@@ -192,7 +228,17 @@ impl Animations {
             }
         }
 
-        // TODO(mrobinson): We need to send animationcancel events.
+        for animation in &set.animations {
+            if animation.state == AnimationState::Canceled {
+                self.add_animation_event(
+                    animation,
+                    TransitionOrAnimationEventType::AnimationCancel,
+                    now,
+                    pipeline_id,
+                );
+            }
+        }
+
         set.clear_canceled_animations();
     }
 
@@ -263,14 +309,21 @@ impl Animations {
         now: f64,
         pipeline_id: PipelineId,
     ) {
+        // Calculate the `elapsed-time` property of the event and take the absolute
+        // value to prevent -0 values.
         let elapsed_time = match event_type {
             TransitionOrAnimationEventType::TransitionRun |
+            TransitionOrAnimationEventType::TransitionStart => transition
+                .property_animation
+                .duration
+                .min((-transition.delay).max(0.)),
             TransitionOrAnimationEventType::TransitionEnd => transition.property_animation.duration,
             TransitionOrAnimationEventType::TransitionCancel => {
                 (now - transition.start_time).max(0.)
             },
             _ => unreachable!(),
-        };
+        }
+        .abs();
 
         self.pending_events
             .borrow_mut()
@@ -291,6 +344,7 @@ impl Animations {
         &self,
         animation: &Animation,
         event_type: TransitionOrAnimationEventType,
+        now: f64,
         pipeline_id: PipelineId,
     ) {
         let num_iterations = match animation.iteration_state {
@@ -298,11 +352,25 @@ impl Animations {
             KeyframesIterationState::Infinite(current) => current,
         };
 
+        let active_duration = match animation.iteration_state {
+            KeyframesIterationState::Finite(_, max) => max * animation.duration,
+            KeyframesIterationState::Infinite(_) => std::f64::MAX,
+        };
+
+        // Calculate the `elapsed-time` property of the event and take the absolute
+        // value to prevent -0 values.
         let elapsed_time = match event_type {
+            TransitionOrAnimationEventType::AnimationStart => {
+                (-animation.delay).max(0.).min(active_duration)
+            },
             TransitionOrAnimationEventType::AnimationIteration |
             TransitionOrAnimationEventType::AnimationEnd => num_iterations * animation.duration,
+            TransitionOrAnimationEventType::AnimationCancel => {
+                (num_iterations * animation.duration) + (now - animation.started_at).max(0.)
+            },
             _ => unreachable!(),
-        };
+        }
+        .abs();
 
         self.pending_events
             .borrow_mut()
@@ -333,10 +401,13 @@ impl Animations {
 
             let event_atom = match event.event_type {
                 TransitionOrAnimationEventType::AnimationEnd => atom!("animationend"),
+                TransitionOrAnimationEventType::AnimationStart => atom!("animationstart"),
+                TransitionOrAnimationEventType::AnimationCancel => atom!("animationcancel"),
                 TransitionOrAnimationEventType::AnimationIteration => atom!("animationiteration"),
                 TransitionOrAnimationEventType::TransitionCancel => atom!("transitioncancel"),
                 TransitionOrAnimationEventType::TransitionEnd => atom!("transitionend"),
                 TransitionOrAnimationEventType::TransitionRun => atom!("transitionrun"),
+                TransitionOrAnimationEventType::TransitionStart => atom!("transitionstart"),
             };
             let parent = EventInit {
                 bubbles: true,
@@ -381,25 +452,39 @@ 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 transitionstart event occurs when a transition’s delay phase ends."
+    TransitionStart,
     /// "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 animationstart event occurs at the start of the animation. If there is
+    /// an animation-delay then this event will fire once the delay period has expired."
+    AnimationStart,
     /// "The animationiteration event occurs at the end of each iteration of an
     /// animation, except when an animationend event would fire at the same time."
     AnimationIteration,
     /// "The animationend event occurs when the animation finishes"
     AnimationEnd,
+    /// "The animationcancel event occurs when the animation stops running in a way
+    /// that does not fire an animationend event..."
+    AnimationCancel,
 }
 
 impl TransitionOrAnimationEventType {
     /// 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 | Self::AnimationIteration => false,
+            Self::TransitionRun |
+            Self::TransitionEnd |
+            Self::TransitionCancel |
+            Self::TransitionStart => true,
+            Self::AnimationEnd |
+            Self::AnimationIteration |
+            Self::AnimationStart |
+            Self::AnimationCancel => false,
         }
     }
 }
diff --git a/components/style/animation.rs b/components/style/animation.rs
index 2330f41f448e..152692a0b286 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -139,17 +139,27 @@ impl PropertyAnimation {
 /// This structure represents the state of an animation.
 #[derive(Clone, Debug, MallocSizeOf, PartialEq)]
 pub enum AnimationState {
+    /// The animation has been created, but is not running yet. This state
+    /// is also used when an animation is still in the first delay phase.
+    Pending,
+    /// This animation is currently running.
+    Running,
     /// This animation is paused. The inner field is the percentage of progress
     /// when it was paused, from 0 to 1.
     Paused(f64),
-    /// This animation is currently running.
-    Running,
     /// This animation has finished.
     Finished,
     /// This animation has been canceled.
     Canceled,
 }
 
+impl AnimationState {
+    /// Whether or not this state requires its owning animation to be ticked.
+    fn needs_to_be_ticked(&self) -> bool {
+        *self == AnimationState::Running || *self == AnimationState::Pending
+    }
+}
+
 /// This structure represents a keyframes animation current iteration state.
 ///
 /// If the iteration count is infinite, there's no other state, otherwise we
@@ -174,7 +184,8 @@ pub struct Animation {
     /// The internal animation from the style system.
     pub keyframes_animation: KeyframesAnimation,
 
-    /// The time this animation started at.
+    /// The time this animation started at, which is the current value of the animation
+    /// timeline when this animation was created plus any animation delay.
     pub started_at: f64,
 
     /// The duration of this animation.
@@ -273,9 +284,11 @@ impl Animation {
     /// canceled due to changes in the style.
     pub fn has_ended(&self, time: f64) -> bool {
         match self.state {
-            AnimationState::Canceled | AnimationState::Paused(_) => return false,
-            AnimationState::Finished => return true,
             AnimationState::Running => {},
+            AnimationState::Finished => return true,
+            AnimationState::Pending | AnimationState::Canceled | AnimationState::Paused(_) => {
+                return false
+            },
         }
 
         if !self.iteration_over(time) {
@@ -312,9 +325,31 @@ impl Animation {
         let old_direction = self.current_direction;
         let old_state = self.state.clone();
         let old_iteration_state = self.iteration_state.clone();
+
         *self = other.clone();
 
-        let mut new_started_at = old_started_at;
+        self.started_at = old_started_at;
+        self.current_direction = old_direction;
+
+        // Don't update the iteration count, just the iteration limit.
+        // TODO: see how changing the limit affects rendering in other browsers.
+        // We might need to keep the iteration count even when it's infinite.
+        match (&mut self.iteration_state, old_iteration_state) {
+            (
+                &mut KeyframesIterationState::Finite(ref mut iters, _),
+                KeyframesIterationState::Finite(old_iters, _),
+            ) => *iters = old_iters,
+            _ => {},
+        }
+
+        // Don't pause or restart animations that should remain finished.
+        // We call mem::replace because `has_ended(...)` looks at `Animation::state`.
+        let new_state = std::mem::replace(&mut self.state, Running);
+        if old_state == Finished && self.has_ended(now) {
+            self.state = Finished;
+        } else {
+            self.state = new_state;
+        }
 
         // If we're unpausing the animation, fake the start time so we seem to
         // restore it.
@@ -322,28 +357,22 @@ impl Animation {
         // If the animation keeps paused, keep the old value.
         //
         // If we're pausing the animation, compute the progress value.
-        match (&mut self.state, old_state) {
-            (&mut Running, Paused(progress)) => new_started_at = now - (self.duration * progress),
-            (&mut Paused(ref mut new), Paused(old)) => *new = old,
-            (&mut Paused(ref mut progress), Running) => {
+        match (&mut self.state, &old_state) {
+            (&mut Pending, &Paused(progress)) => {
+                self.started_at = now - (self.duration * progress);
+            },
+            (&mut Paused(ref mut new), &Paused(old)) => *new = old,
+            (&mut Paused(ref mut progress), &Running) => {
                 *progress = (now - old_started_at) / old_duration
             },
             _ => {},
         }
 
-        // Don't update the iteration count, just the iteration limit.
-        // TODO: see how changing the limit affects rendering in other browsers.
-        // We might need to keep the iteration count even when it's infinite.
-        match (&mut self.iteration_state, old_iteration_state) {
-            (
-                &mut KeyframesIterationState::Finite(ref mut iters, _),
-                KeyframesIterationState::Finite(old_iters, _),
-            ) => *iters = old_iters,
-            _ => {},
+        // Try to detect when we should skip straight to the running phase to
+        // avoid sending multiple animationstart events.
+        if self.state == Pending && self.started_at <= now && old_state != Pending {
+            self.state = Running;
         }
-
-        self.current_direction = old_direction;
-        self.started_at = new_started_at;
     }
 
     /// Update the given style to reflect the values specified by this `Animation`
@@ -360,7 +389,7 @@ impl Animation {
         let started_at = self.started_at;
 
         let now = match self.state {
-            AnimationState::Running | AnimationState::Finished => {
+            AnimationState::Running | AnimationState::Pending | AnimationState::Finished => {
                 context.current_time_for_animations
             },
             AnimationState::Paused(progress) => started_at + duration * progress,
@@ -551,9 +580,12 @@ pub struct Transition {
     pub node: OpaqueNode,
 
     /// The start time of this transition, which is the current value of the animation
-    /// timeline when this transition created.
+    /// timeline when this transition was created plus any animation delay.
     pub start_time: f64,
 
+    /// The delay used for this transition.
+    pub delay: f64,
+
     /// The internal style `PropertyAnimation` for this transition.
     pub property_animation: PropertyAnimation,
 
@@ -724,26 +756,25 @@ impl ElementAnimationSet {
     }
 
     /// Whether or not this state needs animation ticks for its transitions
-    /// or animations. New animations don't need ticks until they are no
-    /// longer marked as new.
+    /// or animations.
     pub fn needs_animation_ticks(&self) -> bool {
         self.animations
             .iter()
-            .any(|animation| animation.state == AnimationState::Running && !animation.is_new) ||
-            self.transitions.iter().any(|transition| {
-                transition.state == AnimationState::Running && !transition.is_new
-            })
+            .any(|animation| animation.state.needs_to_be_ticked()) ||
+            self.transitions
+                .iter()
+                .any(|transition| transition.state.needs_to_be_ticked())
     }
 
     /// The number of running animations and transitions for this `ElementAnimationSet`.
     pub fn running_animation_and_transition_count(&self) -> usize {
         self.animations
             .iter()
-            .filter(|animation| animation.state == AnimationState::Running)
+            .filter(|animation| animation.state.needs_to_be_ticked())
             .count() +
             self.transitions
                 .iter()
-                .filter(|transition| transition.state == AnimationState::Running)
+                .filter(|transition| transition.state.needs_to_be_ticked())
                 .count()
     }
 
@@ -870,8 +901,9 @@ impl ElementAnimationSet {
         let mut new_transition = Transition {
             node: opaque_node,
             start_time: now + delay,
+            delay,
             property_animation,
-            state: AnimationState::Running,
+            state: AnimationState::Pending,
             is_new: true,
             reversing_adjusted_start_value,
             reversing_shortening_factor: 1.0,
@@ -1040,7 +1072,7 @@ pub fn maybe_start_animations<E>(
 
         let state = match box_style.animation_play_state_mod(i) {
             AnimationPlayState::Paused => AnimationState::Paused(0.),
-            AnimationPlayState::Running => AnimationState::Running,
+            AnimationPlayState::Running => AnimationState::Pending,
         };
 
         let new_animation = Animation {
diff --git a/tests/wpt/metadata-layout-2020/css/css-animations/animationevent-types.html.ini b/tests/wpt/metadata-layout-2020/css/css-animations/animationevent-types.html.ini
deleted file mode 100644
index 256ea2378e50..000000000000
--- a/tests/wpt/metadata-layout-2020/css/css-animations/animationevent-types.html.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[animationevent-types.html]
-  expected: TIMEOUT
-  [animationstart event is instanceof AnimationEvent]
-    expected: TIMEOUT
-
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 854c8c6e480b..5913cd5b9364 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
@@ -1,5 +1,5 @@
 [Element-getAnimations.tentative.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26626
   [getAnimations for CSS Animations with animation-name: none]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/css/css-animations/animationevent-pseudoelement.html.ini b/tests/wpt/metadata/css/css-animations/animationevent-pseudoelement.html.ini
index 69cb0d12c276..c7f08fec5f78 100644
--- a/tests/wpt/metadata/css/css-animations/animationevent-pseudoelement.html.ini
+++ b/tests/wpt/metadata/css/css-animations/animationevent-pseudoelement.html.ini
@@ -1,5 +1,5 @@
 [animationevent-pseudoelement.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/10316
   expected: TIMEOUT
   [AnimationEvent should have the correct pseudoElement memeber]
     expected: TIMEOUT
diff --git a/tests/wpt/metadata/css/css-animations/animationevent-types.html.ini b/tests/wpt/metadata/css/css-animations/animationevent-types.html.ini
deleted file mode 100644
index 944fcee5779d..000000000000
--- a/tests/wpt/metadata/css/css-animations/animationevent-types.html.ini
+++ /dev/null
@@ -1,6 +0,0 @@
-[animationevent-types.html]
-  bug: https://github.com/servo/servo/issues/21564
-  expected: TIMEOUT
-  [animationstart event is instanceof AnimationEvent]
-    expected: TIMEOUT
-
diff --git a/tests/wpt/metadata/css/css-transitions/events-006.html.ini b/tests/wpt/metadata/css/css-transitions/events-006.html.ini
index a884e8df5740..edacd476df2b 100644
--- a/tests/wpt/metadata/css/css-transitions/events-006.html.ini
+++ b/tests/wpt/metadata/css/css-transitions/events-006.html.ini
@@ -1,5 +1,5 @@
 [events-006.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/10316
   expected: TIMEOUT
   [transition padding-left on ::after]
     expected: NOTRUN
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-from-to.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-from-to.html.ini
index 79ed388547d1..ac3f2998e7f9 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-from-to.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-from-to.html.ini
@@ -1,5 +1,5 @@
 [variable-animation-from-to.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   expected: TIMEOUT
   [Verify CSS variable value before animation]
     expected: FAIL
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-over-transition.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-over-transition.html.ini
index c6edf5d258b5..6544dd3874eb 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-over-transition.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-over-transition.html.ini
@@ -1,5 +1,5 @@
 [variable-animation-over-transition.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   expected: TIMEOUT
   [Verify CSS variable value before animation]
     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 e61911d3325b..bed87a131592 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
@@ -1,2 +1,2 @@
 [variable-animation-substitute-into-keyframe-shorthand.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
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 77ff1af8a7d3..4722727d2086 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
@@ -1,5 +1,5 @@
 [variable-animation-substitute-into-keyframe-transform.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   [Verify transform before animation]
     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 ccac15c61813..99e8565a805c 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
@@ -1,2 +1,2 @@
 [variable-animation-substitute-into-keyframe.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
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 8fe87ce4aa4e..3290f1660297 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
@@ -1,5 +1,5 @@
 [variable-animation-substitute-within-keyframe-fallback.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   [Verify color after animation]
     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 7046292ccd78..ca4fe4dfc757 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
@@ -1,5 +1,5 @@
 [variable-animation-substitute-within-keyframe-multiple.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   [Verify color after animation]
     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 705cf7ed37ad..d347620f3dcd 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
@@ -1,5 +1,5 @@
 [variable-animation-substitute-within-keyframe.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   [Verify color after animation]
     expected: FAIL
 
diff --git a/tests/wpt/metadata/css/css-variables/variable-animation-to-only.html.ini b/tests/wpt/metadata/css/css-variables/variable-animation-to-only.html.ini
index 5e862e864ff6..976008b1d68a 100644
--- a/tests/wpt/metadata/css/css-variables/variable-animation-to-only.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-animation-to-only.html.ini
@@ -1,5 +1,5 @@
 [variable-animation-to-only.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   expected: TIMEOUT
   [Verify CSS variable value after animation]
     expected: TIMEOUT
diff --git a/tests/wpt/metadata/css/css-variables/variable-transitions-from-no-value.html.ini b/tests/wpt/metadata/css/css-variables/variable-transitions-from-no-value.html.ini
index 1d039975c270..b29d8db3da10 100644
--- a/tests/wpt/metadata/css/css-variables/variable-transitions-from-no-value.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-transitions-from-no-value.html.ini
@@ -1,5 +1,5 @@
 [variable-transitions-from-no-value.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   expected: TIMEOUT
   [Verify CSS variable value after transition]
     expected: NOTRUN
diff --git a/tests/wpt/metadata/css/css-variables/variable-transitions-to-no-value.html.ini b/tests/wpt/metadata/css/css-variables/variable-transitions-to-no-value.html.ini
index b36005d4be8c..a76040845612 100644
--- a/tests/wpt/metadata/css/css-variables/variable-transitions-to-no-value.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-transitions-to-no-value.html.ini
@@ -1,5 +1,5 @@
 [variable-transitions-to-no-value.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   expected: TIMEOUT
   [Verify CSS variable value after transition]
     expected: NOTRUN
diff --git a/tests/wpt/metadata/css/css-variables/variable-transitions-transition-property-variable-before-value.html.ini b/tests/wpt/metadata/css/css-variables/variable-transitions-transition-property-variable-before-value.html.ini
index 53ad4c18ded6..cb48c956d7d1 100644
--- a/tests/wpt/metadata/css/css-variables/variable-transitions-transition-property-variable-before-value.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-transitions-transition-property-variable-before-value.html.ini
@@ -1,5 +1,5 @@
 [variable-transitions-transition-property-variable-before-value.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   expected: TIMEOUT
   [Verify CSS variable value after transition]
     expected: NOTRUN
diff --git a/tests/wpt/metadata/css/css-variables/variable-transitions-value-before-transition-property-variable.html.ini b/tests/wpt/metadata/css/css-variables/variable-transitions-value-before-transition-property-variable.html.ini
index d9b360988cdb..81ca758a5996 100644
--- a/tests/wpt/metadata/css/css-variables/variable-transitions-value-before-transition-property-variable.html.ini
+++ b/tests/wpt/metadata/css/css-variables/variable-transitions-value-before-transition-property-variable.html.ini
@@ -1,5 +1,5 @@
 [variable-transitions-value-before-transition-property-variable.html]
-  bug: https://github.com/servo/servo/issues/21564
+  bug: https://github.com/servo/servo/issues/26625
   expected: TIMEOUT
   [Verify CSS variable value after transition]
     expected: NOTRUN
diff --git a/tests/wpt/mozilla/meta-layout-2020/css/animations/animation-events.html.ini b/tests/wpt/mozilla/meta-layout-2020/css/animations/animation-events.html.ini
new file mode 100644
index 000000000000..33f08914f308
--- /dev/null
+++ b/tests/wpt/mozilla/meta-layout-2020/css/animations/animation-events.html.ini
@@ -0,0 +1,2 @@
+prefs: ["layout.animations.test.enabled:false",
+        "dom.testbinding.enabled:false"]
diff --git a/tests/wpt/mozilla/meta-layout-2020/css/animations/transition-events.html.ini b/tests/wpt/mozilla/meta-layout-2020/css/animations/transition-events.html.ini
new file mode 100644
index 000000000000..f53fd6a50504
--- /dev/null
+++ b/tests/wpt/mozilla/meta-layout-2020/css/animations/transition-events.html.ini
@@ -0,0 +1,3 @@
+prefs: ["layout.animations.test.enabled:false",
+        "dom.testbinding.enabled:false"]
+
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index 70c41cd6def6..f5225f39757a 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -12849,6 +12849,15 @@
    },
    "css": {
     "animations": {
+     "animation-events.html": [
+      "0975aa64ec47ca4b4c8fc1e0a40414a51719ad67",
+      [
+       null,
+       {
+        "timeout": "long"
+       }
+      ]
+     ],
      "animation-fill-mode.html": [
       "4cfaab9fbce0adccd83f592935e63fa8ff58a1cf",
       [
@@ -12884,6 +12893,13 @@
        {}
       ]
      ],
+     "transition-events.html": [
+      "b561fc8353276e6bdd13a9d1b965f57733ecd19b",
+      [
+       null,
+       {}
+      ]
+     ],
      "transition-raf.html": [
       "c38404503408e04b3c75b42df18ec3a7ec0819f5",
       [
diff --git a/tests/wpt/mozilla/meta/css/animations/animation-events.html.ini b/tests/wpt/mozilla/meta/css/animations/animation-events.html.ini
new file mode 100644
index 000000000000..33f08914f308
--- /dev/null
+++ b/tests/wpt/mozilla/meta/css/animations/animation-events.html.ini
@@ -0,0 +1,2 @@
+prefs: ["layout.animations.test.enabled:false",
+        "dom.testbinding.enabled:false"]
diff --git a/tests/wpt/mozilla/meta/css/animations/transition-events.html.ini b/tests/wpt/mozilla/meta/css/animations/transition-events.html.ini
new file mode 100644
index 000000000000..f53fd6a50504
--- /dev/null
+++ b/tests/wpt/mozilla/meta/css/animations/transition-events.html.ini
@@ -0,0 +1,3 @@
+prefs: ["layout.animations.test.enabled:false",
+        "dom.testbinding.enabled:false"]
+
diff --git a/tests/wpt/mozilla/tests/css/animations/animation-events.html b/tests/wpt/mozilla/tests/css/animations/animation-events.html
new file mode 100644
index 000000000000..0975aa64ec47
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/animations/animation-events.html
@@ -0,0 +1,160 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>CSS animation event dispatch</title>
+<meta name="timeout" content="long">
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#event-dispatch"/>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+@keyframes anim {
+  from { margin-left: 0px; }
+  to { margin-left: 100px; }
+}
+</style>
+<div id="log"></div>
+<script>
+'use strict';
+
+// This series of tests is a forked version of the Web Platform Test
+// /css/css-animations/event-dispatch.tentative that do not make use
+// of the Web Animations API, since Servo doesn't yet support Web Animations.
+
+function waitForFrame() {
+  return new Promise(resolve => {
+    window.requestAnimationFrame(resolve);
+  });
+}
+
+// All animation events should be received on the next animation frame.
+const animationEventsTimeout = () => {
+    return new Promise(function(resolve) {
+        setTimeout(resolve, 5000);
+    });
+}
+
+const setupAnimation = (t, animationStyle) => {
+  var div = document.createElement('div');
+  div.setAttribute('style', 'animation: ' + animationStyle);
+  document.body.appendChild(div);
+  if (t && typeof t.add_cleanup === 'function') {
+    t.add_cleanup(function() {
+      if (div.parentNode) {
+        div.remove();
+      }
+    });
+  }
+
+  const watcher = new EventWatcher(t, div, [ 'animationstart',
+                                             'animationiteration',
+                                             'animationend',
+                                             'animationcancel' ],
+                                   animationEventsTimeout);
+
+  return { watcher, div };
+};
+
+promise_test(async t => {
+  const { watcher } = setupAnimation(t, 'anim 100s');
+
+  const events = await watcher.wait_for(
+    ['animationstart' ],
+    {
+      record: 'all',
+    }
+  );
+  assert_equals(events[0].elapsedTime, 0.0);
+}, 'animationstart');
+
+promise_test(async t => {
+  const { watcher } = setupAnimation(t, 'anim 0.1s');
+
+  const events = await watcher.wait_for(
+    ['animationstart', 'animationend'],
+    {
+      record: 'all',
+    }
+  );
+  assert_equals(events[0].elapsedTime, 0);
+  assert_equals(events[0].animationName, "anim");
+
+  assert_approx_equals(events[1].elapsedTime, 0.1, 0.001);
+  assert_equals(events[1].animationName, "anim");
+}, 'animationstart and animationend');
+
+promise_test(async t => {
+  const { watcher } = setupAnimation(t, 'anim 1s 0.5s');
+
+  const events = await watcher.wait_for(
+    ['animationstart', 'animationend'], { record: 'all', }
+  );
+  assert_approx_equals(events[0].elapsedTime, 0, 0.01);
+  assert_equals(events[0].animationName, "anim");
+
+  assert_approx_equals(events[1].elapsedTime, 1, 0.01);
+  assert_equals(events[1].animationName, "anim");
+}, 'animationstart and animationend with positive delay');
+
+promise_test(async t => {
+  const { watcher } = setupAnimation(t, 'anim 100s -99.99s');
+
+  const events = await watcher.wait_for(
+    ['animationstart', 'animationend'], { record: 'all', }
+  );
+  assert_approx_equals(events[0].elapsedTime, 99.99, 0.1);
+  assert_equals(events[0].animationName, "anim");
+
+  assert_approx_equals(events[1].elapsedTime, 99.99, 0.1);
+  assert_equals(events[1].animationName, "anim");
+}, 'animationstart and animationend with negative delay');
+
+promise_test(async t => {
+  const { watcher } = setupAnimation(t, 'anim 100s -200s');
+
+  const events = await watcher.wait_for(
+    ['animationstart', 'animationend'], { record: 'all', }
+  );
+  assert_approx_equals(events[0].elapsedTime, 99.99, 0.1);
+  assert_equals(events[0].animationName, "anim");
+
+  assert_approx_equals(events[1].elapsedTime, 99.99, 0.1);
+  assert_equals(events[1].animationName, "anim");
+}, 'animationstart and animationend with negative delay larger than active duration');
+
+promise_test(async t => {
+  const { watcher, div } = setupAnimation(t, 'anim 100s');
+  await watcher.wait_for('animationstart');
+
+  div.style.animation = "";
+
+  await watcher.wait_for('animationcancel');
+}, 'animationcancel');
+
+promise_test(async t => {
+  const { watcher, div } = setupAnimation(t, 'anim 100s 50s');
+
+  // Wait for two animation frames. One is not enough in some browser engines.
+  await waitForFrame();
+  await waitForFrame();
+
+  div.style.animation = "";
+  const events = await watcher.wait_for(
+    ['animationcancel'], { record: 'all', }
+  );
+
+  assert_equals(events[0].elapsedTime, 0);
+}, 'animationcancel with positive delay');
+
+promise_test(async t => {
+  const { watcher, div } = setupAnimation(t, 'anim 100s -50s');
+  await watcher.wait_for('animationstart');
+
+  div.style.animation = "";
+
+  const events = await watcher.wait_for(
+    ['animationcancel'], { record: 'all', }
+  );
+
+  assert_approx_equals(events[0].elapsedTime, 50, 0.1);
+}, 'animationcancel with negative delay');
+
+</script>
diff --git a/tests/wpt/mozilla/tests/css/animations/transition-events.html b/tests/wpt/mozilla/tests/css/animations/transition-events.html
new file mode 100644
index 000000000000..b561fc835327
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/animations/transition-events.html
@@ -0,0 +1,131 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>CSS transition event dispatch</title>
+<link rel="help" href="https://drafts.csswg.org/css-transitions-2/#event-dispatch">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+'use strict';
+
+// This series of tests is a forked version of the Web Platform Test
+// /css/css-transitions/event-dispatch.tentative that do not make use
+// of the Web Animations API, since Servo doesn't yet support Web Animations.
+
+
+// All transition events should be received on the next animation frame.
+function transitionEventsTimeout() {
+  return new Promise(function(resolve) {
+      return new Promise(resolve => {
+        window.requestAnimationFrame(resolve);
+      });
+   });
+};
+
+const setupTransition = (t, transitionStyle) => {
+  var div = document.createElement('div');
+  div.setAttribute('style', 'transition: ' + transitionStyle);
+  document.body.appendChild(div);
+  if (t && typeof t.add_cleanup === 'function') {
+    t.add_cleanup(function() {
+      if (div.parentNode) {
+        div.remove();
+      }
+    });
+  }
+  const watcher = new EventWatcher(t, div, [ 'transitionrun',
+                                             'transitionstart',
+                                             'transitionend',
+                                             'transitioncancel' ],
+                                   transitionEventsTimeout);
+  getComputedStyle(div).marginLeft;
+
+  div.style.marginLeft = '100px';
+
+  return { watcher, div };
+};
+
+promise_test(async t => {
+  const { watcher } = setupTransition(t, 'margin-left 100s 100s');
+  const events = await watcher.wait_for(
+    ['transitionrun' ],
+    {
+      record: 'all',
+    }
+  );
+  assert_equals(events[0].elapsedTime, 0.0);
+}, 'transitionrun');
+
+promise_test(async t => {
+  const { watcher } = setupTransition(t, 'margin-left 100s');
+  const events = await watcher.wait_for(
+    ['transitionrun', 'transitionstart', ],
+    {
+      record: 'all',
+    }
+  );
+  assert_equals(events[0].elapsedTime, 0.0);
+  assert_equals(events[1].elapsedTime, 0.0);
+}, 'transitionrun, transitionstart');
+
+promise_test(async t => {
+  const { watcher, div } = setupTransition(t, 'margin-left 0.1s');
+
+  const events = await watcher.wait_for(
+    ['transitionrun', 'transitionstart', 'transitionend'],
+    {
+      record: 'all',
+    }
+  );
+  assert_equals(events[0].elapsedTime, 0);
+  assert_equals(events[0].propertyName, "margin-left");
+
+  assert_equals(events[1].elapsedTime, 0);
+  assert_equals(events[1].propertyName, "margin-left");
+
+  assert_approx_equals(events[2].elapsedTime, 0.1, 0.01);
+  assert_equals(events[2].propertyName, "margin-left");
+
+}, 'transitionrun, transitionstart, transitionend');
+
+promise_test(async t => {
+  const { watcher, div } = setupTransition(t, 'margin-left 1s 0.5s');
+
+  const events = await watcher.wait_for(
+    ['transitionrun', 'transitionstart', 'transitionend'],
+    {
+      record: 'all',
+    }
+  );
+
+  assert_equals(events[0].elapsedTime, 0);
+  assert_equals(events[0].propertyName, "margin-left");
+
+  assert_equals(events[1].elapsedTime, 0);
+  assert_equals(events[1].propertyName, "margin-left");
+
+  assert_equals(events[2].elapsedTime, 1);
+  assert_equals(events[2].propertyName, "margin-left");
+}, 'transitionrun, transitionstart, transitionend with positive delay');
+
+
+promise_test(async t => {
+  const { watcher, div } = setupTransition(t, 'margin-left 100s -99.99s');
+
+  const events = await watcher.wait_for(
+    ['transitionrun', 'transitionstart', 'transitionend'],
+    {
+      record: 'all',
+    }
+  );
+  assert_approx_equals(events[0].elapsedTime, 99.99, 0.1);
+  assert_equals(events[0].propertyName, "margin-left");
+
+  assert_approx_equals(events[1].elapsedTime, 99.99, 0.1);
+  assert_equals(events[1].propertyName, "margin-left");
+
+  assert_approx_equals(events[2].elapsedTime, 99.99, 0.1);
+  assert_equals(events[2].propertyName, "margin-left");
+}, 'transitionrun, transitionstart, transitionend with negative delay');
+
+</script>