--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -3,16 +3,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/dom/HTMLMediaElement.h"
 #include "mozilla/dom/HTMLMediaElementBinding.h"
 #include "mozilla/dom/HTMLSourceElement.h"
 #include "mozilla/dom/ElementInlines.h"
+#include "mozilla/dom/Promise.h"
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/MathAlgorithms.h"
 #include "mozilla/AsyncEventDispatcher.h"
 #include "mozilla/dom/MediaEncryptedEvent.h"
 #include "mozilla/EMEUtils.h"
 
 #include "base/basictypes.h"
 #include "nsIDOMHTMLMediaElement.h"
@@ -148,16 +149,32 @@ static const double THRESHOLD_HIGH_PLAYB
 static const double THRESHOLD_LOW_PLAYBACKRATE_AUDIO = 0.5;
 
 // Media error values.  These need to match the ones in MediaError.webidl.
 static const unsigned short MEDIA_ERR_ABORTED = 1;
 static const unsigned short MEDIA_ERR_NETWORK = 2;
 static const unsigned short MEDIA_ERR_DECODE = 3;
 static const unsigned short MEDIA_ERR_SRC_NOT_SUPPORTED = 4;
 
+static void
+ResolvePromisesWithUndefined(const nsTArray<RefPtr<Promise>>& aPromises)
+{
+  for (auto& promise : aPromises) {
+    promise->MaybeResolveWithUndefined();
+  }
+}
+
+static void
+RejectPromises(const nsTArray<RefPtr<Promise>>& aPromises, nsresult aError)
+{
+  for (auto& promise : aPromises) {
+    promise->MaybeReject(aError);
+  }
+}
+
 // Under certain conditions there may be no-one holding references to
 // a media element from script, DOM parent, etc, but the element may still
 // fire meaningful events in the future so we can't destroy it yet:
 // 1) If the element is delaying the load event (or would be, if it were
 // in a document), then events up to loadeddata or error could be fired,
 // so we need to stay alive.
 // 2) If the element is not paused and playback has not ended, then
 // we will (or might) play, sending timeupdate and ended events and possibly
@@ -238,16 +255,84 @@ public:
     // Silently cancel if our load has been cancelled.
     if (IsCancelled())
       return NS_OK;
 
     return mElement->DispatchEvent(mName);
   }
 };
 
+/*
+ * If no error is passed while constructing an instance, the instance will
+ * resolve the passed promises with undefined; otherwise, the instance will
+ * reject the passed promises with the passed error.
+ *
+ * The constructor appends the constructed instance into the passed media
+ * element's mPendingPlayPromisesRunners member and once the the runner is run
+ * (whether fulfilled or canceled), it removes itself from
+ * mPendingPlayPromisesRunners.
+ */
+class HTMLMediaElement::nsResolveOrRejectPendingPlayPromisesRunner : public nsMediaEvent
+{
+  nsTArray<RefPtr<Promise>> mPromises;
+  nsresult mError;
+
+public:
+  nsResolveOrRejectPendingPlayPromisesRunner(HTMLMediaElement* aElement,
+                                             nsTArray<RefPtr<Promise>>&& aPromises,
+                                             nsresult aError = NS_OK)
+  : nsMediaEvent(aElement)
+  , mPromises(Move(aPromises))
+  , mError(aError)
+  {
+    mElement->mPendingPlayPromisesRunners.AppendElement(this);
+  }
+
+  void ResolveOrReject()
+  {
+    if (NS_SUCCEEDED(mError)) {
+      ResolvePromisesWithUndefined(mPromises);
+    } else {
+      RejectPromises(mPromises, mError);
+    }
+  }
+
+  NS_IMETHOD Run() override
+  {
+    if (!IsCancelled()) {
+      ResolveOrReject();
+    }
+
+    mElement->mPendingPlayPromisesRunners.RemoveElement(this);
+    return NS_OK;
+  }
+};
+
+class HTMLMediaElement::nsNotifyAboutPlayingRunner : public nsResolveOrRejectPendingPlayPromisesRunner
+{
+public:
+  nsNotifyAboutPlayingRunner(HTMLMediaElement* aElement,
+                             nsTArray<RefPtr<Promise>>&& aPendingPlayPromises)
+  : nsResolveOrRejectPendingPlayPromisesRunner(aElement,
+                                               Move(aPendingPlayPromises))
+  {
+  }
+
+  NS_IMETHOD Run() override
+  {
+    if (IsCancelled()) {
+      mElement->mPendingPlayPromisesRunners.RemoveElement(this);
+      return NS_OK;
+    }
+
+    mElement->DispatchEvent(NS_LITERAL_STRING("playing"));
+    return nsResolveOrRejectPendingPlayPromisesRunner::Run();
+  }
+};
+
 class nsSourceErrorEventRunner : public nsMediaEvent
 {
 private:
   nsCOMPtr<nsIContent> mSource;
 public:
   nsSourceErrorEventRunner(HTMLMediaElement* aElement,
                            nsIContent* aSource)
     : nsMediaEvent(aElement),
@@ -1276,16 +1361,17 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_
     NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mOutputStreams[i].mStream);
   }
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPlayed);
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mTextTrackManager)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mAudioTrackList)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mVideoTrackList)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mMediaKeys)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mSelectedVideoStreamTrack)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPendingPlayPromises)
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(HTMLMediaElement, nsGenericHTMLElement)
   if (tmp->mSrcStream) {
     // Need to EndMediaStreamPlayback to clear mSrcStream and make sure everything
     // gets unhooked correctly.
     tmp->EndSrcMediaStreamPlayback();
   }
@@ -1304,16 +1390,17 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_IN
     NS_IMPL_CYCLE_COLLECTION_UNLINK(mOutputStreams[i].mStream)
   }
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mPlayed)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mTextTrackManager)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mAudioTrackList)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mVideoTrackList)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mMediaKeys)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mSelectedVideoStreamTrack)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mPendingPlayPromises)
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(HTMLMediaElement)
   NS_INTERFACE_MAP_ENTRY(nsIDOMHTMLMediaElement)
 NS_INTERFACE_MAP_END_INHERITING(nsGenericHTMLElement)
 
 // nsIDOMHTMLMediaElement
 NS_IMPL_URI_ATTR(HTMLMediaElement, Src, src)
@@ -5683,17 +5770,24 @@ nsresult HTMLMediaElement::DispatchAsync
 
   // Save events that occur while in the bfcache. These will be dispatched
   // if the page comes out of the bfcache.
   if (mEventDeliveryPaused) {
     mPendingEvents.AppendElement(aName);
     return NS_OK;
   }
 
-  nsCOMPtr<nsIRunnable> event = new nsAsyncEventRunner(aName, this);
+  nsCOMPtr<nsIRunnable> event;
+
+  if (aName.EqualsLiteral("playing")) {
+    event = new nsNotifyAboutPlayingRunner(this, TakePendingPlayPromises());
+  } else {
+    event = new nsAsyncEventRunner(aName, this);
+  }
+
   OwnerDoc()->Dispatch("HTMLMediaElement::DispatchAsyncEvent",
                        TaskCategory::Other,
                        event.forget());
 
   if ((aName.EqualsLiteral("play") || aName.EqualsLiteral("playing"))) {
     mPlayTime.Start();
     if (IsHidden()) {
       HiddenVideoStart();
@@ -6910,10 +7004,58 @@ void
 HTMLMediaElement::UpdateCustomPolicyAfterPlayed()
 {
   OpenUnsupportedMediaWithExternalAppIfNeeded();
   if (mAudioChannelWrapper) {
     mAudioChannelWrapper->NotifyPlayStarted();
   }
 }
 
+nsTArray<RefPtr<Promise>>
+HTMLMediaElement::TakePendingPlayPromises()
+{
+  return Move(mPendingPlayPromises);
+}
+
+void
+HTMLMediaElement::NotifyAboutPlaying()
+{
+  // Stick to the DispatchAsyncEvent() call path for now because we want to
+  // trigger some telemetry-related codes in the DispatchAsyncEvent() method.
+  DispatchAsyncEvent(NS_LITERAL_STRING("playing"));
+}
+
+void
+HTMLMediaElement::AsyncResolvePendingPlayPromises()
+{
+  if (mShuttingDown) {
+    return;
+  }
+
+  nsCOMPtr<nsIRunnable> event
+    = new nsResolveOrRejectPendingPlayPromisesRunner(this,
+                                                     TakePendingPlayPromises());
+
+  OwnerDoc()->Dispatch("HTMLMediaElement::AsyncResolvePendingPlayPromises",
+                       TaskCategory::Other,
+                       event.forget());
+}
+
+void
+HTMLMediaElement::AsyncRejectPendingPlayPromises(nsresult aError)
+{
+  if (mShuttingDown) {
+    return;
+  }
+
+  nsCOMPtr<nsIRunnable> event
+    = new nsResolveOrRejectPendingPlayPromisesRunner(this,
+                                                     TakePendingPlayPromises(),
+                                                     aError);
+
+  OwnerDoc()->Dispatch("HTMLMediaElement::AsyncRejectPendingPlayPromises",
+                       TaskCategory::Other,
+                       event.forget());
+}
+
+
 } // namespace dom
 } // namespace mozilla

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -11,17 +11,16 @@
 #include "nsGenericHTMLElement.h"
 #include "MediaDecoderOwner.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsIObserver.h"
 #include "mozilla/CORSMode.h"
 #include "DecoderTraits.h"
 #include "nsIAudioChannelAgent.h"
 #include "mozilla/Attributes.h"
-#include "mozilla/dom/Promise.h"
 #include "mozilla/dom/TextTrackManager.h"
 #include "mozilla/WeakPtr.h"
 #include "MediaDecoder.h"
 #include "mozilla/dom/MediaKeys.h"
 #include "mozilla/StateWatching.h"
 #include "nsGkAtoms.h"
 #include "PrincipalChangeObserver.h"
 
@@ -68,16 +67,17 @@ class nsRange;
 namespace mozilla {
 namespace dom {
 
 // Number of milliseconds between timeupdate events as defined by spec
 #define TIMEUPDATE_MS 250
 
 class MediaError;
 class MediaSource;
+class Promise;
 class TextTrackList;
 class AudioTrackList;
 class VideoTrackList;
 
 class HTMLMediaElement : public nsGenericHTMLElement,
                          public nsIDOMHTMLMediaElement,
                          public MediaDecoderOwner,
                          public PrincipalChangeObserver<DOMMediaStream>,
@@ -1226,23 +1226,41 @@ protected:
 
   // If the network state is empty and then we would trigger DoLoad().
   void MaybeDoLoad();
 
   // Anything we need to check after played success and not related with spec.
   void UpdateCustomPolicyAfterPlayed();
 
   class nsAsyncEventRunner;
+  class nsNotifyAboutPlayingRunner;
+  class nsResolveOrRejectPendingPlayPromisesRunner;
   using nsGenericHTMLElement::DispatchEvent;
   // For nsAsyncEventRunner.
   nsresult DispatchEvent(const nsAString& aName);
 
   // Open unsupported types media with the external app when the media element
   // triggers play() after loaded fail. eg. preload the data before start play.
   void OpenUnsupportedMediaWithExternalAppIfNeeded() const;
+  
+  // This method moves the mPendingPlayPromises into a temperate object. So the
+  // mPendingPlayPromises is cleared after this method call.
+  nsTArray<RefPtr<Promise>> TakePendingPlayPromises();
+
+  // This method snapshots the mPendingPlayPromises by TakePendingPlayPromises()
+  // and queues a task to resolve them.
+  void AsyncResolvePendingPlayPromises();
+
+  // This method snapshots the mPendingPlayPromises by TakePendingPlayPromises()
+  // and queues a task to reject them.
+  void AsyncRejectPendingPlayPromises(nsresult aError);
+
+  // This method snapshots the mPendingPlayPromises by TakePendingPlayPromises()
+  // and queues a task to resolve them also to dispatch a "playing" event.
+  void NotifyAboutPlaying();
 
   // The current decoder. Load() has been called on this decoder.
   // At most one of mDecoder and mSrcStream can be non-null.
   RefPtr<MediaDecoder> mDecoder;
 
   // Observers listening to changes to the mDecoder principal.
   // Used by streams captured from this element.
   nsTArray<DecoderPrincipalChangeObserver*> mDecoderPrincipalChangeObservers;
@@ -1666,14 +1684,25 @@ private:
   Visibility mVisibilityState;
 
   UniquePtr<ErrorSink> mErrorSink;
 
   // This wrapper will handle all audio channel related stuffs, eg. the operations
   // of tab audio indicator, Fennec's media control.
   // Note: mAudioChannelWrapper might be null after GC happened.
   RefPtr<AudioChannelAgentCallback> mAudioChannelWrapper;
+  
+  // A list of pending play promises. The elements are pushed during the play()
+  // method call and are resolved/rejected during further playback steps.
+  nsTArray<RefPtr<Promise>> mPendingPlayPromises;
+
+  // A list of already-dispatched but not yet run
+  // nsResolveOrRejectPendingPlayPromisesRunners.
+  // Runners whose Run() method is called remove themselves from this list.
+  // We keep track of these because the load algorithm resolves/rejects all
+  // already-dispatched pending play promises.
+  nsTArray<nsResolveOrRejectPendingPlayPromisesRunner*> mPendingPlayPromisesRunners;
 };
 
 } // namespace dom
 } // namespace mozilla
 
 #endif // mozilla_dom_HTMLMediaElement_h