diff --git a/dom/animation/Animation.cpp b/dom/animation/Animation.cpp
index 1014fa16982..e424764c212 100644
--- a/dom/animation/Animation.cpp
+++ b/dom/animation/Animation.cpp
@@ -444,7 +444,7 @@ Animation::DoCancel()
   mHoldTime.SetNull();
   mStartTime.SetNull();
 
-  UpdateEffect();
+  UpdateTiming(SeekFlag::NoSeek);
 }
 
 void
diff --git a/layout/style/nsAnimationManager.cpp b/layout/style/nsAnimationManager.cpp
index 6356a772157..1c952179c95 100644
--- a/layout/style/nsAnimationManager.cpp
+++ b/layout/style/nsAnimationManager.cpp
@@ -420,6 +420,8 @@ nsAnimationManager::CheckAnimationRule(nsStyleContext* aStyleContext,
           }
         }
 
+        oldAnim->CopyAnimationIndex(*newAnim->AsCSSAnimation());
+
         if (animationChanged) {
           nsNodeUtils::AnimationChanged(oldAnim);
         }
@@ -553,6 +555,7 @@ nsAnimationManager::BuildAnimations(nsStyleContext* aStyleContext,
 
     nsRefPtr<CSSAnimation> dest = new CSSAnimation(aTimeline, src.GetName());
     dest->SetOwningElement(*aTarget, aStyleContext->GetPseudoType());
+    dest->SetAnimationIndex(static_cast<uint64_t>(animIdx));
     aAnimations.AppendElement(dest);
 
     AnimationTiming timing;
diff --git a/layout/style/nsAnimationManager.h b/layout/style/nsAnimationManager.h
index ae2237a5074..9196a858f0c 100644
--- a/layout/style/nsAnimationManager.h
+++ b/layout/style/nsAnimationManager.h
@@ -101,10 +101,25 @@ public:
     mOwningPseudoType = nsCSSPseudoElements::ePseudo_NotPseudoElement;
 
     Animation::CancelFromStyle();
+    MOZ_ASSERT(mSequenceNum == kUnsequenced);
   }
 
   bool IsStylePaused() const { return mIsStylePaused; }
 
+  bool IsUsingCustomCompositeOrder() const override { return !!mOwningElement; }
+
+  void SetAnimationIndex(uint64_t aIndex)
+  {
+    MOZ_ASSERT(IsUsingCustomCompositeOrder());
+    mSequenceNum = aIndex;
+  }
+  void CopyAnimationIndex(const CSSAnimation& aOther)
+  {
+    MOZ_ASSERT(IsUsingCustomCompositeOrder() &&
+               aOther.IsUsingCustomCompositeOrder());
+    mSequenceNum = aOther.mSequenceNum;
+  }
+
   void QueueEvents(EventArray& aEventsToDispatch);
 
   // Returns the element or pseudo-element whose animation-name property