--- a/dom/events/CompositionEvent.cpp
+++ b/dom/events/CompositionEvent.cpp
@@ -33,16 +33,34 @@ CompositionEvent::CompositionEvent(Event
     mEvent->mFlags.mCancelable = false;
   }
 
   // XXX Do we really need to duplicate the data value?
   mData = mEvent->AsCompositionEvent()->mData;
   // TODO: Native event should have locale information.
 }
 
+// static
+already_AddRefed<CompositionEvent>
+CompositionEvent::Constructor(const GlobalObject& aGlobal,
+                     const nsAString& aType,
+                     const CompositionEventInit& aParam,
+                     ErrorResult& aRv)
+{
+  nsCOMPtr<EventTarget> t = do_QueryInterface(aGlobal.GetAsSupports());
+  RefPtr<CompositionEvent> e = new CompositionEvent(t, nullptr, nullptr);
+  bool trusted = e->Init(t);
+  e->InitCompositionEvent(aType, aParam.mBubbles, aParam.mCancelable,
+                          aParam.mView, aParam.mData, EmptyString());
+  e->mDetail = aParam.mDetail;
+  e->SetTrusted(trusted);
+  e->SetComposed(aParam.mComposed);
+  return e.forget();
+}
+
 NS_IMPL_ADDREF_INHERITED(CompositionEvent, UIEvent)
 NS_IMPL_RELEASE_INHERITED(CompositionEvent, UIEvent)
 
 NS_INTERFACE_MAP_BEGIN(CompositionEvent)
 NS_INTERFACE_MAP_END_INHERITING(UIEvent)
 
 void
 CompositionEvent::GetData(nsAString& aData) const

--- a/dom/events/CompositionEvent.h
+++ b/dom/events/CompositionEvent.h
@@ -23,16 +23,21 @@ class CompositionEvent : public UIEvent
 public:
   CompositionEvent(EventTarget* aOwner,
                    nsPresContext* aPresContext,
                    WidgetCompositionEvent* aEvent);
 
   NS_DECL_ISUPPORTS_INHERITED
   NS_FORWARD_TO_UIEVENT
 
+  static already_AddRefed<CompositionEvent> Constructor(const GlobalObject& aGlobal,
+                                                        const nsAString& aType,
+                                                        const CompositionEventInit& aParam,
+                                                        ErrorResult& aRv);
+
   virtual JSObject* WrapObjectInternal(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) override
   {
     return CompositionEventBinding::Wrap(aCx, this, aGivenProto);
   }
 
   void InitCompositionEvent(const nsAString& aType,
                             bool aCanBubble,
                             bool aCancelable,

--- a/dom/webidl/CompositionEvent.webidl
+++ b/dom/webidl/CompositionEvent.webidl
@@ -4,29 +4,34 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * http://dev.w3.org/2006/webapi/DOM-Level-3-Events/html/DOM3-Events.html#events-CompositionEvent
  *
  * Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C
  * liability, trademark and document use rules apply.
  */
 
+[Constructor(DOMString type, optional CompositionEventInit eventInitDict)]
 interface CompositionEvent : UIEvent
 {
   readonly attribute DOMString? data;
   readonly attribute DOMString  locale;
 
  /**
   * ranges is trying to expose TextRangeArray in Gecko so a
   * js-plugin couble be able to know the clauses information
   */
   [ChromeOnly,Cached,Pure]
   readonly attribute sequence<TextClause> ranges;
 };
 
+dictionary CompositionEventInit : UIEventInit {
+  DOMString data = "";
+};
+
 partial interface CompositionEvent
 {
   void initCompositionEvent(DOMString typeArg,
                             boolean canBubbleArg,
                             boolean cancelableArg,
                             Window? viewArg,
                             DOMString? dataArg,
                             DOMString localeArg);

deleted file mode 100644
--- a/testing/web-platform/meta/dom/events/Event-subclasses-constructors.html.ini
+++ /dev/null
@@ -1,20 +0,0 @@
-[Event-subclasses-constructors.html]
-  type: testharness
-  [CompositionEvent constructor (no argument)]
-    expected: FAIL
-
-  [CompositionEvent constructor (undefined argument)]
-    expected: FAIL
-
-  [CompositionEvent constructor (null argument)]
-    expected: FAIL
-
-  [CompositionEvent constructor (empty argument)]
-    expected: FAIL
-
-  [CompositionEvent constructor (argument with default values)]
-    expected: FAIL
-
-  [CompositionEvent constructor (argument with non-default values)]
-    expected: FAIL
-