--- a/dom/animation/Animation.h
+++ b/dom/animation/Animation.h
@@ -93,16 +93,20 @@ public:
   static already_AddRefed<Animation>
   Constructor(const GlobalObject& aGlobal,
               KeyframeEffectReadOnly* aEffect,
               const Optional<AnimationTimeline*>& aTimeline,
               ErrorResult& aRv);
   void GetId(nsAString& aResult) const { aResult = mId; }
   void SetId(const nsAString& aId);
   KeyframeEffectReadOnly* GetEffect() const { return mEffect; }
+  void SetEffect(AnimationEffectReadOnly* aEffect)
+  {
+    // TODO: Merged with SetEffect(KeyframeEffectReadOnly*) in the next patch.
+  }
   void SetEffect(KeyframeEffectReadOnly* aEffect);
   AnimationTimeline* GetTimeline() const { return mTimeline; }
   void SetTimeline(AnimationTimeline* aTimeline);
   Nullable<TimeDuration> GetStartTime() const { return mStartTime; }
   void SetStartTime(const Nullable<TimeDuration>& aNewStartTime);
   Nullable<TimeDuration> GetCurrentTime() const;
   void SetCurrentTime(const TimeDuration& aNewCurrentTime);
   double PlaybackRate() const { return mPlaybackRate; }

--- a/dom/webidl/Animation.webidl
+++ b/dom/webidl/Animation.webidl
@@ -12,19 +12,18 @@
 
 enum AnimationPlayState { "idle", "pending", "running", "paused", "finished" };
 
 [Func="nsDocument::IsElementAnimateEnabled",
  Constructor (optional KeyframeEffectReadOnly? effect = null,
               optional AnimationTimeline? timeline)]
 interface Animation : EventTarget {
   attribute DOMString id;
-  // Bug 1049975: Make 'effect' writeable
   [Func="nsDocument::IsWebAnimationsEnabled", Pure]
-  readonly attribute AnimationEffectReadOnly? effect;
+  attribute AnimationEffectReadOnly? effect;
   [Func="nsDocument::IsWebAnimationsEnabled"]
   attribute AnimationTimeline? timeline;
   [BinaryName="startTimeAsDouble"]
   attribute double? startTime;
   [SetterThrows, BinaryName="currentTimeAsDouble"]
   attribute double? currentTime;
 
            attribute double             playbackRate;