--- a/dom/animation/KeyframeEffectReadOnly.h
+++ b/dom/animation/KeyframeEffectReadOnly.h
@@ -115,16 +115,18 @@ struct Keyframe
   nsTArray<PropertyValuePair>   mPropertyValues;
 };
 
 struct AnimationPropertySegment
 {
   float mFromKey, mToKey;
   StyleAnimationValue mFromValue, mToValue;
   Maybe<ComputedTimingFunction> mTimingFunction;
+  dom::CompositeOperation mFromComposite = dom::CompositeOperation::Replace;
+  dom::CompositeOperation mToComposite = dom::CompositeOperation::Replace;
 
   bool operator==(const AnimationPropertySegment& aOther) const
   {
     return mFromKey == aOther.mFromKey &&
            mToKey == aOther.mToKey &&
            mFromValue == aOther.mFromValue &&
            mToValue == aOther.mToValue &&
            mTimingFunction == aOther.mTimingFunction;
@@ -176,16 +178,29 @@ struct AnimationProperty
   {
     return mProperty == aOther.mProperty &&
            mSegments == aOther.mSegments;
   }
   bool operator!=(const AnimationProperty& aOther) const
   {
     return !(*this == aOther);
   }
+
+  // Returns true if there is at least one segment with a composite value that
+  // is not 'replace'.
+  bool HasNonReplaceComposite() const
+  {
+    for (const AnimationPropertySegment& segment : mSegments) {
+      if (segment.mFromComposite != dom::CompositeOperation::Replace ||
+          segment.mToComposite != dom::CompositeOperation::Replace) {
+        return true;
+      }
+    }
+    return false;
+  }
 };
 
 struct ElementPropertyTransition;
 
 namespace dom {
 
 class Animation;