diff --git a/dom/media/mediasession/MediaSession.cpp b/dom/media/mediasession/MediaSession.cpp
index ea632c61d203..761ea255e4a6 100644
--- a/dom/media/mediasession/MediaSession.cpp
+++ b/dom/media/mediasession/MediaSession.cpp
@@ -96,21 +96,19 @@ void MediaSession::DispatchNotifyHandler(
              const MediaSessionActionDetails& aDetails)
         : mozilla::Runnable("MediaSession::DispatchNotifyHandler"),
           mSession(aSession),
-          mAction(aDetails.mAction) {}
+          mDetails(aDetails) {}
 
     MOZ_CAN_RUN_SCRIPT_BOUNDARY NS_IMETHOD Run() override {
       if (RefPtr<MediaSessionActionHandler> handler =
-              mSession->GetActionHandler(mAction)) {
-        MediaSessionActionDetails details;
-        details.mAction = mAction;
-        handler->Call(details);
+              mSession->GetActionHandler(mDetails.mAction)) {
+        handler->Call(mDetails);
       }
       return NS_OK;
     }
 
    private:
     RefPtr<const MediaSession> mSession;
-    MediaSessionAction mAction;
+    MediaSessionActionDetails mDetails;
   };
 
   RefPtr<nsIRunnable> runnable = new Runnable(this, aDetails);
diff --git a/dom/media/mediasession/test/test_setactionhandler.html b/dom/media/mediasession/test/test_setactionhandler.html
index f41d5658ae73..92fa6444852c 100644
--- a/dom/media/mediasession/test/test_setactionhandler.html
+++ b/dom/media/mediasession/test/test_setactionhandler.html
@@ -13,6 +13,8 @@ SimpleTest.waitForExplicitFinish();
 const ACTIONS = [
   "play",
   "pause",
+  "seekbackward",
+  "seekforward",
   "previoustrack",
   "nexttrack",
   "stop",
@@ -55,6 +57,9 @@ function generateAction(action) {
 
 function generateActionDetails(action) {
   let details = { action };
+  if (action == "seekbackward" || action == "seekforward") {
+    details.seekOffset = 3.14159;
+  }
   return details;
 }
 
diff --git a/dom/webidl/MediaSession.webidl b/dom/webidl/MediaSession.webidl
index c13096edc591..9df8e72e73b7 100644
--- a/dom/webidl/MediaSession.webidl
+++ b/dom/webidl/MediaSession.webidl
@@ -13,10 +13,12 @@ enum MediaSessionPlaybackState {
   "playing"
 };
 
-// TODO: Implement the missing seek* (bug 1580623) and skipad (bug 1582569) actions
+// TODO: Implement the missing seekto (bug 1621403) and skipad (bug 1582569) actions
 enum MediaSessionAction {
   "play",
   "pause",
+  "seekbackward",
+  "seekforward",
   "previoustrack",
   "nexttrack",
   "stop",
@@ -68,6 +70,9 @@ dictionary MediaImage {
 
 dictionary MediaSessionActionDetails {
   required MediaSessionAction action;
+  // Merge MediaSessionSeekActionDetails here:
+  // https://github.com/w3c/mediasession/issues/234
+  double seekOffset;
 };
 
 // TODO: Implement MediaPositionState (bug 1582509)
diff --git a/testing/web-platform/meta/mediasession/setactionhandler.html.ini b/testing/web-platform/meta/mediasession/setactionhandler.html.ini
index 57ec81cba051..17488f285533 100644
--- a/testing/web-platform/meta/mediasession/setactionhandler.html.ini
+++ b/testing/web-platform/meta/mediasession/setactionhandler.html.ini
@@ -1,5 +1,4 @@
 prefs: [dom.media.mediasession.enabled:true]
-[setactionhandler.html] # Bug 1580623
+[setactionhandler.html] # Bug 1621403
   [Test that setActionHandler() can be executed for supported actions]
     expected: FAIL
-