diff --git a/dom/animation/KeyframeEffect.cpp b/dom/animation/KeyframeEffect.cpp
index c23e9e6d74d0..0374253ce0c9 100644
--- a/dom/animation/KeyframeEffect.cpp
+++ b/dom/animation/KeyframeEffect.cpp
@@ -221,9 +221,11 @@ void
 KeyframeEffectReadOnly::GetComputedTimingAsDict(
     ComputedTimingProperties& aRetVal) const
 {
+  double playbackRate = mAnimation ? mAnimation->PlaybackRate() : 1;
   const Nullable<TimeDuration> currentTime = GetLocalTime();
   GetComputedTimingDictionary(GetComputedTimingAt(currentTime,
-                                                  SpecifiedTiming()),
+                                                  SpecifiedTiming(),
+                                                  playbackRate),
                               currentTime,
                               SpecifiedTiming(),
                               aRetVal);
@@ -232,7 +234,8 @@ KeyframeEffectReadOnly::GetComputedTimingAsDict(
 ComputedTiming
 KeyframeEffectReadOnly::GetComputedTimingAt(
     const Nullable<TimeDuration>& aLocalTime,
-    const TimingParams& aTiming)
+    const TimingParams& aTiming,
+    double aPlaybackRate)
 {
   const StickyTimeDuration zeroDuration;
 
@@ -271,9 +274,15 @@ KeyframeEffectReadOnly::GetComputedTimingAt(
   // Calculate the time within the active interval.
   // https://w3c.github.io/web-animations/#active-time
   StickyTimeDuration activeTime;
-  if (localTime >=
-        std::min(StickyTimeDuration(aTiming.mDelay + result.mActiveDuration),
-                 result.mEndTime)) {
+
+  StickyTimeDuration beforeActiveBoundary =
+    std::min(StickyTimeDuration(aTiming.mDelay), result.mEndTime);
+  StickyTimeDuration activeAfterBoundary =
+    std::min(StickyTimeDuration(aTiming.mDelay + result.mActiveDuration),
+             result.mEndTime);
+
+  if (localTime > activeAfterBoundary ||
+      (aPlaybackRate >= 0 && localTime == activeAfterBoundary)) {
     result.mPhase = ComputedTiming::AnimationPhase::After;
     if (!result.FillsForwards()) {
       // The animation isn't active or filling at this time.
@@ -282,8 +291,8 @@ KeyframeEffectReadOnly::GetComputedTimingAt(
     activeTime = std::max(std::min(result.mActiveDuration,
                                    result.mActiveDuration + aTiming.mEndDelay),
                           zeroDuration);
-  } else if (localTime <
-               std::min(StickyTimeDuration(aTiming.mDelay), result.mEndTime)) {
+  } else if (localTime < beforeActiveBoundary ||
+             (aPlaybackRate < 0 && localTime == beforeActiveBoundary)) {
     result.mPhase = ComputedTiming::AnimationPhase::Before;
     if (!result.FillsBackwards()) {
       // The animation isn't active or filling at this time.
@@ -391,6 +400,15 @@ KeyframeEffectReadOnly::GetComputedTimingAt(
   return result;
 }
 
+ComputedTiming
+KeyframeEffectReadOnly::GetComputedTiming(const TimingParams* aTiming) const
+{
+  double playbackRate = mAnimation ? mAnimation->PlaybackRate() : 1;
+  return GetComputedTimingAt(GetLocalTime(),
+                             aTiming ? *aTiming : SpecifiedTiming(),
+                             playbackRate);
+}
+
 // https://w3c.github.io/web-animations/#in-play
 bool
 KeyframeEffectReadOnly::IsInPlay() const
diff --git a/dom/animation/KeyframeEffect.h b/dom/animation/KeyframeEffect.h
index be1171138098..a7ba2d53305a 100644
--- a/dom/animation/KeyframeEffect.h
+++ b/dom/animation/KeyframeEffect.h
@@ -264,16 +264,13 @@ public:
   // (because it is not currently active and is not filling at this time).
   static ComputedTiming
   GetComputedTimingAt(const Nullable<TimeDuration>& aLocalTime,
-                      const TimingParams& aTiming);
+                      const TimingParams& aTiming,
+                      double aPlaybackRate);
 
   // Shortcut for that gets the computed timing using the current local time as
   // calculated from the timeline time.
   ComputedTiming
-  GetComputedTiming(const TimingParams* aTiming = nullptr) const
-  {
-    return GetComputedTimingAt(GetLocalTime(),
-                               aTiming ? *aTiming : SpecifiedTiming());
-  }
+  GetComputedTiming(const TimingParams* aTiming = nullptr) const;
 
   void
   GetComputedTimingAsDict(ComputedTimingProperties& aRetVal) const override;
diff --git a/gfx/layers/composite/AsyncCompositionManager.cpp b/gfx/layers/composite/AsyncCompositionManager.cpp
index 37b6d1f4930a..ad55aa88ba0e 100644
--- a/gfx/layers/composite/AsyncCompositionManager.cpp
+++ b/gfx/layers/composite/AsyncCompositionManager.cpp
@@ -652,7 +652,8 @@ SampleAnimations(Layer* aLayer, TimeStamp aPoint)
 
           ComputedTiming computedTiming =
             dom::KeyframeEffectReadOnly::GetComputedTimingAt(
-              Nullable<TimeDuration>(elapsedDuration), timing);
+              Nullable<TimeDuration>(elapsedDuration), timing,
+              animation.playbackRate());
 
           MOZ_ASSERT(!computedTiming.mProgress.IsNull(),
                      "iteration progress should not be null");
diff --git a/layout/style/nsTransitionManager.cpp b/layout/style/nsTransitionManager.cpp
index d5e1b1b91306..8987a0017510 100644
--- a/layout/style/nsTransitionManager.cpp
+++ b/layout/style/nsTransitionManager.cpp
@@ -91,7 +91,8 @@ ElementPropertyTransition::UpdateStartValueFromReplacedTransition()
                                          TimeStamp::Now(),
                                          mReplacedTransition->mStartTime,
                                          mReplacedTransition->mPlaybackRate),
-    mReplacedTransition->mTiming);
+    mReplacedTransition->mTiming,
+    mReplacedTransition->mPlaybackRate);
 
   if (!computedTiming.mProgress.IsNull()) {
     double valuePosition =
diff --git a/testing/web-platform/meta/web-animations/timing-model/animation-effects/phases-and-states.html.ini b/testing/web-platform/meta/web-animations/timing-model/animation-effects/phases-and-states.html.ini
deleted file mode 100644
index 2df07f090d13..000000000000
--- a/testing/web-platform/meta/web-animations/timing-model/animation-effects/phases-and-states.html.ini
+++ /dev/null
@@ -1,5 +0,0 @@
-[phases-and-states.html]
-  type: testharness
-  [Phase calculation for a simple animation effect with negative playback rate]
-    expected: FAIL
-    bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1286476