--- a/dom/animation/Animation.h
+++ b/dom/animation/Animation.h
@@ -256,20 +256,16 @@ public:
   TimeStamp AnimationTimeToTimeStamp(const StickyTimeDuration& aTime) const;
 
   bool IsPausedOrPausing() const
   {
     return PlayState() == AnimationPlayState::Paused ||
            mPendingState == PendingState::PausePending;
   }
 
-  bool HasInPlayEffect() const
-  {
-    return GetEffect() && GetEffect()->IsInPlay();
-  }
   bool HasCurrentEffect() const
   {
     return GetEffect() && GetEffect()->IsCurrent();
   }
   bool IsInEffect() const
   {
     return GetEffect() && GetEffect()->IsInEffect();
   }

--- a/dom/animation/AnimationEffectReadOnly.cpp
+++ b/dom/animation/AnimationEffectReadOnly.cpp
@@ -39,27 +39,16 @@ NS_INTERFACE_MAP_END
 AnimationEffectReadOnly::AnimationEffectReadOnly(
   nsIDocument* aDocument, AnimationEffectTimingReadOnly* aTiming)
   : mDocument(aDocument)
   , mTiming(aTiming)
 {
   MOZ_ASSERT(aTiming);
 }
 
-// https://w3c.github.io/web-animations/#in-play
-bool
-AnimationEffectReadOnly::IsInPlay() const
-{
-  if (!mAnimation || mAnimation->PlayState() == AnimationPlayState::Finished) {
-    return false;
-  }
-
-  return GetComputedTiming().mPhase == ComputedTiming::AnimationPhase::Active;
-}
-
 // https://w3c.github.io/web-animations/#current
 bool
 AnimationEffectReadOnly::IsCurrent() const
 {
   if (!mAnimation || mAnimation->PlayState() == AnimationPlayState::Finished) {
     return false;
   }
 

--- a/dom/animation/AnimationEffectReadOnly.h
+++ b/dom/animation/AnimationEffectReadOnly.h
@@ -44,17 +44,16 @@ public:
   virtual ElementPropertyTransition* AsTransition() { return nullptr; }
   virtual const ElementPropertyTransition* AsTransition() const
   {
     return nullptr;
   }
 
   nsISupports* GetParentObject() const { return mDocument; }
 
-  bool IsInPlay() const;
   bool IsCurrent() const;
   bool IsInEffect() const;
   bool IsActiveDurationZero() const
   {
     return SpecifiedTiming().ActiveDuration() == StickyTimeDuration();
   }
 
   already_AddRefed<AnimationEffectTimingReadOnly> Timing();