diff --git a/dom/base/nsDOMTokenList.h b/dom/base/nsDOMTokenList.h
index 6a9c5f4c632b..891f78d51cb1 100644
--- a/dom/base/nsDOMTokenList.h
+++ b/dom/base/nsDOMTokenList.h
@@ -75,7 +75,7 @@ public:
   void Remove(const nsAString& aToken, mozilla::ErrorResult& aError);
   void Remove(const nsTArray<nsString>& aTokens,
               mozilla::ErrorResult& aError);
-  void Replace(const nsAString& aToken,
+  bool Replace(const nsAString& aToken,
                const nsAString& aNewToken,
                mozilla::ErrorResult& aError);
   bool Toggle(const nsAString& aToken,
@@ -99,7 +99,7 @@ protected:
                    const nsTArray<nsString>& aTokens);
   void RemoveInternal(const nsAttrValue* aAttr,
                       const nsTArray<nsString>& aTokens);
-  void ReplaceInternal(const nsAttrValue* aAttr,
+  bool ReplaceInternal(const nsAttrValue* aAttr,
                        const nsAString& aToken,
                        const nsAString& aNewToken);
   inline const nsAttrValue* GetParsedAttr();
diff --git a/dom/events/Event.cpp b/dom/events/Event.cpp
index bfac28150726..e43a7abafa90 100644
--- a/dom/events/Event.cpp
+++ b/dom/events/Event.cpp
@@ -246,6 +246,17 @@ Event::GetTarget(nsIDOMEventTarget** aTarget)
   return NS_OK;
 }
 
+bool
+Event::IsSrcElementEnabled(JSContext* /* unused */, JSObject* /* unused */)
+{
+  // Not a pref, because that's a pain on workers.
+#ifdef NIGHTLY_BUILD
+  return true;
+#else
+  return false;
+#endif
+}
+
 EventTarget*
 Event::GetCurrentTarget() const
 {
diff --git a/dom/events/Event.h b/dom/events/Event.h
index 242747cf66ff..a24db29fb355 100644
--- a/dom/events/Event.h
+++ b/dom/events/Event.h
@@ -171,6 +171,7 @@ public:
   // void GetType(nsString& aRetval) {}
 
   EventTarget* GetTarget() const;
+  static bool IsSrcElementEnabled(JSContext* /* unused */, JSObject* /* unused */);
   EventTarget* GetCurrentTarget() const;
 
   void ComposedPath(nsTArray<RefPtr<EventTarget>>& aPath);
diff --git a/dom/webidl/Event.webidl b/dom/webidl/Event.webidl
index 23820e698df9..d3172c6f48df 100644
--- a/dom/webidl/Event.webidl
+++ b/dom/webidl/Event.webidl
@@ -17,6 +17,8 @@ interface Event {
   readonly attribute DOMString type;
   [Pure]
   readonly attribute EventTarget? target;
+  [Pure, BinaryName="target", Func="Event::IsSrcElementEnabled"]
+  readonly attribute EventTarget? srcElement;
   [Pure]
   readonly attribute EventTarget? currentTarget;