--- a/dom/animation/KeyframeEffect.cpp
+++ b/dom/animation/KeyframeEffect.cpp
@@ -42,16 +42,25 @@ KeyframeEffect::Constructor(
     const UnrestrictedDoubleOrKeyframeEffectOptions& aOptions,
     ErrorResult& aRv)
 {
   return ConstructKeyframeEffect<KeyframeEffect>(aGlobal, aTarget, aKeyframes,
                                                  aOptions, aRv);
 }
 
 /* static */ already_AddRefed<KeyframeEffect>
+KeyframeEffect::Constructor(const GlobalObject& aGlobal,
+                            KeyframeEffectReadOnly& aSource,
+                            ErrorResult& aRv)
+{
+  // TODO: Implement this in the later patches.
+  return nullptr;
+}
+
+/* static */ already_AddRefed<KeyframeEffect>
 KeyframeEffect::Constructor(
     const GlobalObject& aGlobal,
     const Nullable<ElementOrCSSPseudoElement>& aTarget,
     JS::Handle<JSObject*> aKeyframes,
     const UnrestrictedDoubleOrKeyframeAnimationOptions& aOptions,
     ErrorResult& aRv)
 {
   return ConstructKeyframeEffect<KeyframeEffect>(aGlobal, aTarget, aKeyframes,

--- a/dom/animation/KeyframeEffect.h
+++ b/dom/animation/KeyframeEffect.h
@@ -42,16 +42,21 @@ public:
 
   static already_AddRefed<KeyframeEffect>
   Constructor(const GlobalObject& aGlobal,
               const Nullable<ElementOrCSSPseudoElement>& aTarget,
               JS::Handle<JSObject*> aKeyframes,
               const UnrestrictedDoubleOrKeyframeEffectOptions& aOptions,
               ErrorResult& aRv);
 
+  static already_AddRefed<KeyframeEffect>
+  Constructor(const GlobalObject& aGlobal,
+              KeyframeEffectReadOnly& aSource,
+              ErrorResult& aRv);
+
   // Variant of Constructor that accepts a KeyframeAnimationOptions object
   // for use with for Animatable.animate.
   // Not exposed to content.
   static already_AddRefed<KeyframeEffect>
   Constructor(const GlobalObject& aGlobal,
               const Nullable<ElementOrCSSPseudoElement>& aTarget,
               JS::Handle<JSObject*> aKeyframes,
               const UnrestrictedDoubleOrKeyframeAnimationOptions& aOptions,

--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -711,16 +711,25 @@ KeyframeEffectReadOnly::Constructor(
     const UnrestrictedDoubleOrKeyframeEffectOptions& aOptions,
     ErrorResult& aRv)
 {
   return ConstructKeyframeEffect<KeyframeEffectReadOnly>(aGlobal, aTarget,
                                                          aKeyframes, aOptions,
                                                          aRv);
 }
 
+/* static */ already_AddRefed<KeyframeEffectReadOnly>
+KeyframeEffectReadOnly::Constructor(const GlobalObject& aGlobal,
+                                    KeyframeEffectReadOnly& aSource,
+                                    ErrorResult& aRv)
+{
+  // TODO: Implement this in the later patches.
+  return nullptr;
+}
+
 void
 KeyframeEffectReadOnly::GetTarget(
     Nullable<OwningElementOrCSSPseudoElement>& aRv) const
 {
   if (!mTarget) {
     aRv.SetNull();
     return;
   }

--- a/dom/animation/KeyframeEffectReadOnly.h
+++ b/dom/animation/KeyframeEffectReadOnly.h
@@ -193,16 +193,21 @@ public:
   // KeyframeEffectReadOnly interface
   static already_AddRefed<KeyframeEffectReadOnly>
   Constructor(const GlobalObject& aGlobal,
               const Nullable<ElementOrCSSPseudoElement>& aTarget,
               JS::Handle<JSObject*> aKeyframes,
               const UnrestrictedDoubleOrKeyframeEffectOptions& aOptions,
               ErrorResult& aRv);
 
+  static already_AddRefed<KeyframeEffectReadOnly>
+  Constructor(const GlobalObject& aGlobal,
+              KeyframeEffectReadOnly& aSource,
+              ErrorResult& aRv);
+
   void GetTarget(Nullable<OwningElementOrCSSPseudoElement>& aRv) const;
   Maybe<NonOwningAnimationTarget> GetTarget() const
   {
     Maybe<NonOwningAnimationTarget> result;
     if (mTarget) {
       result.emplace(*mTarget);
     }
     return result;

--- a/dom/webidl/KeyframeEffect.webidl
+++ b/dom/webidl/KeyframeEffect.webidl
@@ -20,30 +20,28 @@ dictionary KeyframeEffectOptions : Anima
   CompositeOperation          composite = "replace";
   DOMString                   spacing = "distribute";
 };
 
 // Bug 1241783: For the constructor we use (Element or CSSPseudoElement)? for
 // the first argument since we cannot convert a mixin into a union type
 // automatically.
 [Func="nsDocument::IsWebAnimationsEnabled",
- Constructor((Element or CSSPseudoElement)? target,
-             object? keyframes,
-             optional (unrestricted double or KeyframeEffectOptions) options)]
+ Constructor ((Element or CSSPseudoElement)? target,
+              object? keyframes,
+              optional (unrestricted double or KeyframeEffectOptions) options),
+ Constructor (KeyframeEffectReadOnly source)]
 interface KeyframeEffectReadOnly : AnimationEffectReadOnly {
   // Bug 1241783: As with the constructor, we use (Element or CSSPseudoElement)?
   // for the type of |target| instead of Animatable?
   readonly attribute (Element or CSSPseudoElement)?  target;
   readonly attribute IterationCompositeOperation iterationComposite;
   readonly attribute CompositeOperation          composite;
   readonly attribute DOMString                   spacing;
 
-  // Not yet implemented:
-  // KeyframeEffect             clone();
-
   // We use object instead of ComputedKeyframe so that we can put the
   // property-value pairs on the object.
   [Throws] sequence<object> getKeyframes();
 };
 
 // Non-standard extensions
 dictionary AnimationPropertyValueDetails {
   required double             offset;
@@ -61,17 +59,18 @@ dictionary AnimationPropertyDetails {
 
 partial interface KeyframeEffectReadOnly {
   [ChromeOnly, Throws] sequence<AnimationPropertyDetails> getProperties();
 };
 
 [Func="nsDocument::IsWebAnimationsEnabled",
  Constructor ((Element or CSSPseudoElement)? target,
               object? keyframes,
-              optional (unrestricted double or KeyframeEffectOptions) options)]
+              optional (unrestricted double or KeyframeEffectOptions) options),
+ Constructor (KeyframeEffectReadOnly source)]
 interface KeyframeEffect : KeyframeEffectReadOnly {
   inherit attribute (Element or CSSPseudoElement)? target;
   inherit attribute IterationCompositeOperation    iterationComposite;
   // Bug 1216844 - implement additive animation
   // inherit attribute CompositeOperation          composite;
   [SetterThrows]
   inherit attribute DOMString                   spacing;
   [Throws]