--- a/dom/media/eme/MediaKeySession.cpp
+++ b/dom/media/eme/MediaKeySession.cpp
@@ -95,22 +95,16 @@ MediaKeySession::~MediaKeySession()
 
 MediaKeyError*
 MediaKeySession::GetError() const
 {
   return mMediaKeyError;
 }
 
 void
-MediaKeySession::GetKeySystem(nsString& aOutKeySystem) const
-{
-  aOutKeySystem.Assign(mKeySystem);
-}
-
-void
 MediaKeySession::GetSessionId(nsString& aSessionId) const
 {
   aSessionId = GetSessionId();
 }
 
 const nsString&
 MediaKeySession::GetSessionId() const
 {

--- a/dom/media/eme/MediaKeySession.h
+++ b/dom/media/eme/MediaKeySession.h
@@ -48,18 +48,16 @@ public:
 
   JSObject* WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto) override;
 
   // Mark this as resultNotAddRefed to return raw pointers
   MediaKeyError* GetError() const;
 
   MediaKeyStatusMap* KeyStatuses() const;
 
-  void GetKeySystem(nsString& aRetval) const;
-
   void GetSessionId(nsString& aRetval) const;
 
   const nsString& GetSessionId() const;
 
   // Number of ms since epoch at which expiration occurs, or NaN if unknown.
   // TODO: The type of this attribute is still under contention.
   // https://www.w3.org/Bugs/Public/show_bug.cgi?id=25902
   double Expiration() const;

--- a/dom/webidl/MediaKeySession.webidl
+++ b/dom/webidl/MediaKeySession.webidl
@@ -11,17 +11,16 @@
  */
 
 [Pref="media.eme.apiVisible"]
 interface MediaKeySession : EventTarget {
   // error state
   readonly attribute MediaKeyError? error;
 
   // session properties
-  readonly attribute DOMString keySystem;
   readonly attribute DOMString sessionId;
 
   readonly attribute unrestricted double expiration;
 
   readonly attribute Promise<void> closed;
 
   readonly attribute MediaKeyStatusMap keyStatuses;