diff --git a/dom/animation/Animation.cpp b/dom/animation/Animation.cpp
index 352e562a10a..47d8fbcaa8d 100644
--- a/dom/animation/Animation.cpp
+++ b/dom/animation/Animation.cpp
@@ -229,6 +229,18 @@ Animation::ActiveDuration(const AnimationTiming& aTiming)
     aTiming.mIterationDuration.MultDouble(aTiming.mIterationCount));
 }
 
+// http://w3c.github.io/web-animations/#in-play
+bool
+Animation::IsInPlay(const AnimationPlayer& aPlayer) const
+{
+  if (IsFinishedTransition() ||
+      aPlayer.PlayState() == AnimationPlayState::Finished) {
+    return false;
+  }
+
+  return GetComputedTiming().mPhase == ComputedTiming::AnimationPhase_Active;
+}
+
 // http://w3c.github.io/web-animations/#current
 bool
 Animation::IsCurrent(const AnimationPlayer& aPlayer) const
diff --git a/dom/animation/Animation.h b/dom/animation/Animation.h
index cb9c270231c..45a028a5372 100644
--- a/dom/animation/Animation.h
+++ b/dom/animation/Animation.h
@@ -302,6 +302,7 @@ public:
     mIsFinishedTransition = true;
   }
 
+  bool IsInPlay(const AnimationPlayer& aPlayer) const;
   bool IsCurrent(const AnimationPlayer& aPlayer) const;
   bool IsInEffect() const;
 
diff --git a/dom/animation/AnimationPlayer.h b/dom/animation/AnimationPlayer.h
index c4955179fde..4c4e8e2d7a8 100644
--- a/dom/animation/AnimationPlayer.h
+++ b/dom/animation/AnimationPlayer.h
@@ -203,6 +203,10 @@ public:
   }
   bool IsRunning() const;
 
+  bool HasInPlaySource() const
+  {
+    return GetSource() && GetSource()->IsInPlay(*this);
+  }
   bool HasCurrentSource() const
   {
     return GetSource() && GetSource()->IsCurrent(*this);