--- a/dom/media/eme/MediaKeyMessageEvent.cpp
+++ b/dom/media/eme/MediaKeyMessageEvent.cpp
@@ -80,25 +80,20 @@ MediaKeyMessageEvent::Constructor(const 
                                   const nsAString& aType,
                                   const MediaKeyMessageEventInit& aEventInitDict,
                                   ErrorResult& aRv)
 {
   nsCOMPtr<EventTarget> owner = do_QueryInterface(aGlobal.GetAsSupports());
   RefPtr<MediaKeyMessageEvent> e = new MediaKeyMessageEvent(owner);
   bool trusted = e->Init(owner);
   e->InitEvent(aType, aEventInitDict.mBubbles, aEventInitDict.mCancelable);
-  const uint8_t* data = nullptr;
-  size_t length = 0;
-  if (aEventInitDict.mMessage.WasPassed()) {
-    const auto& a = aEventInitDict.mMessage.Value();
-    a.ComputeLengthAndData();
-    data = a.Data();
-    length = a.Length();
-  }
-  e->mMessage = ArrayBuffer::Create(aGlobal.Context(), length, data);
+  aEventInitDict.mMessage.ComputeLengthAndData();
+  e->mMessage = ArrayBuffer::Create(aGlobal.Context(),
+                                    aEventInitDict.mMessage.Length(),
+                                    aEventInitDict.mMessage.Data());
   if (!e->mMessage) {
     aRv.Throw(NS_ERROR_OUT_OF_MEMORY);
     return nullptr;
   }
   e->mMessageType = aEventInitDict.mMessageType;
   e->SetTrusted(trusted);
   e->SetComposed(aEventInitDict.mComposed);
   return e.forget();

--- a/dom/webidl/MediaKeyMessageEvent.webidl
+++ b/dom/webidl/MediaKeyMessageEvent.webidl
@@ -12,19 +12,19 @@
 
 enum MediaKeyMessageType {
   "license-request",
   "license-renewal",
   "license-release",
   "individualization-request"
 };
 
-[Pref="media.eme.apiVisible", Constructor(DOMString type, optional MediaKeyMessageEventInit eventInitDict)]
+[Pref="media.eme.apiVisible", Constructor(DOMString type, MediaKeyMessageEventInit eventInitDict)]
 interface MediaKeyMessageEvent : Event {
   readonly attribute MediaKeyMessageType messageType;
   [Throws]
   readonly attribute ArrayBuffer message;
 };
 
 dictionary MediaKeyMessageEventInit : EventInit {
-  MediaKeyMessageType messageType = "license-request";
-  ArrayBuffer message;
+  required MediaKeyMessageType messageType;
+  required ArrayBuffer message;
 };

--- a/testing/web-platform/tests/encrypted-media/EncryptedMediaExtensions.idl
+++ b/testing/web-platform/tests/encrypted-media/EncryptedMediaExtensions.idl
@@ -1,14 +1,15 @@
 // Encrypted Media Extensions WebIDL
 //
 // NOTE: Please update the link below to the specification version from
 //       which this IDL was extracted.
 //
 // https://www.w3.org/TR/2016/WD-encrypted-media-20160610/
+//  + commit 5499821932391ae2c2e53756ae7ab9fae89d5863
 //
 
 partial interface Navigator {
     Promise<MediaKeySystemAccess> requestMediaKeySystemAccess (DOMString keySystem, sequence<MediaKeySystemConfiguration> supportedConfigurations);
 };
 
 enum MediaKeysRequirement {
     "required",
@@ -81,36 +82,36 @@ enum MediaKeyStatus {
 
 enum MediaKeyMessageType {
     "license-request",
     "license-renewal",
     "license-release",
     "individualization-request"
 };
 
-[ Constructor (DOMString type, optional MediaKeyMessageEventInit eventInitDict)]
+[Constructor(DOMString type, MediaKeyMessageEventInit eventInitDict)]
 interface MediaKeyMessageEvent : Event {
     readonly        attribute MediaKeyMessageType messageType;
     readonly        attribute ArrayBuffer         message;
 };
 
 dictionary MediaKeyMessageEventInit : EventInit {
-             MediaKeyMessageType messageType = "license-request";
-             ArrayBuffer         message;
+             required MediaKeyMessageType messageType;
+             required ArrayBuffer         message;
 };
 
 // partial interface HTMLMediaElement : EventTarget {
 partial interface HTMLMediaElement {
     readonly        attribute MediaKeys?   mediaKeys;
                     attribute EventHandler onencrypted;
                     attribute EventHandler onwaitingforkey;
     Promise<void> setMediaKeys (MediaKeys? mediaKeys);
 };
 
-[ Constructor (DOMString type, optional MediaEncryptedEventInit eventInitDict)]
+[Constructor(DOMString type, optional MediaEncryptedEventInit eventInitDict)]
 interface MediaEncryptedEvent : Event {
     readonly        attribute DOMString    initDataType;
     readonly        attribute ArrayBuffer? initData;
 };
 
 dictionary MediaEncryptedEventInit : EventInit {
              DOMString    initDataType = "";
              ArrayBuffer? initData = null;