--- a/layout/style/nsAnimationManager.cpp
+++ b/layout/style/nsAnimationManager.cpp
@@ -423,16 +423,55 @@ public:
                                          aKeyframes);
   }
   void SetKeyframes(KeyframeEffectReadOnly& aEffect,
                     nsTArray<Keyframe>&& aKeyframes)
   {
     aEffect.SetKeyframes(Move(aKeyframes), mStyleContext);
   }
 
+  // Currently all the animation building code in this file is based on
+  // assumption that creating and removing animations should *not* trigger
+  // additional restyles since those changes will be handled within the same
+  // restyle.
+  //
+  // While that is true for the Gecko style backend, it is not true for the
+  // Servo style backend where we want restyles to be triggered so that we
+  // perform a second animation restyle where we will incorporate the changes
+  // arising from creating and removing animations.
+  //
+  // Fortunately, our attempts to avoid posting extra restyles as part of the
+  // processing here are imperfect and most of the time we happen to post
+  // them anyway. Occasionally, however, we don't. For example, we don't post
+  // a restyle when we create a new animation whose an animation index matches
+  // the default value it was given already (which is typically only true when
+  // the CSSAnimation we create is the first Animation created in a particular
+  // content process).
+  //
+  // As a result, when we are using the Servo backend, whenever we have an added
+  // or removed animation we need to explicitly trigger a restyle.
+  //
+  // This code should eventually disappear along with the Gecko style backend
+  // and we should simply call Play() / Pause() / Cancel() etc. which will
+  // post the required restyles.
+  void NotifyNewOrRemovedAnimation(const Animation& aAnimation)
+  {
+    AnimationEffectReadOnly* effect = aAnimation.GetEffect();
+    if (!effect) {
+      return;
+    }
+
+    KeyframeEffectReadOnly* keyframeEffect = effect->AsKeyframeEffect();
+    if (!keyframeEffect) {
+      return;
+    }
+
+    keyframeEffect->RequestRestyle(EffectCompositor::RestyleType::Standard);
+  }
+
 private:
   const ServoStyleContext* mStyleContext;
 };
 
 class MOZ_STACK_CLASS GeckoCSSAnimationBuilder final {
 public:
   GeckoCSSAnimationBuilder(GeckoStyleContext* aStyleContext,
                            const NonOwningAnimationTarget& aTarget)
@@ -447,16 +486,18 @@ public:
                       const StyleAnimation& aSrc,
                       nsTArray<Keyframe>& aKeyframs);
   void SetKeyframes(KeyframeEffectReadOnly& aEffect,
                     nsTArray<Keyframe>&& aKeyframes)
   {
     aEffect.SetKeyframes(Move(aKeyframes), mStyleContext);
   }
 
+  void NotifyNewOrRemovedAnimation(const Animation&) {}
+
 private:
   nsTArray<Keyframe> BuildAnimationFrames(nsPresContext* aPresContext,
                                           const StyleAnimation& aSrc,
                                           const nsCSSKeyframesRule* aRule);
   Maybe<ComputedTimingFunction> GetKeyframeTimingFunction(
     nsPresContext* aPresContext,
     nsCSSKeyframeRule* aKeyframeRule,
     const Maybe<ComputedTimingFunction>& aInheritedTimingFunction);
@@ -600,16 +641,18 @@ BuildAnimation(nsPresContext* aPresConte
   animation->SetEffectNoUpdate(effect);
 
   if (isStylePaused) {
     animation->PauseFromStyle();
   } else {
     animation->PlayFromStyle();
   }
 
+  aBuilder.NotifyNewOrRemovedAnimation(*animation);
+
   return animation.forget();
 }
 
 bool
 GeckoCSSAnimationBuilder::BuildKeyframes(nsPresContext* aPresContext,
                                          const StyleAnimation& aSrc,
                                          nsTArray<Keyframe>& aKeyframes)
 {
@@ -1094,11 +1137,12 @@ nsAnimationManager::DoUpdateAnimations(
     if (createdCollection) {
       AddElementCollection(collection);
     }
   }
   collection->mAnimations.SwapElements(newAnimations);
 
   // Cancel removed animations
   for (size_t newAnimIdx = newAnimations.Length(); newAnimIdx-- != 0; ) {
+    aBuilder.NotifyNewOrRemovedAnimation(*newAnimations[newAnimIdx]);
     newAnimations[newAnimIdx]->CancelFromStyle();
   }
 }