--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -2517,16 +2517,22 @@ HTMLMediaElement::ResumeLoad(PreloadActi
     // We were loading from a child <source> element. Try to resume the
     // load of that child, and if that fails, try the next child.
     if (NS_FAILED(LoadResource())) {
       LoadFromSourceChildren();
     }
   }
 }
 
+bool
+HTMLMediaElement::AllowedToPlay() const
+{
+  return AutoplayPolicy::IsAllowedToPlay(*this) == nsIAutoplay::ALLOWED;
+}
+
 void
 HTMLMediaElement::UpdatePreloadAction()
 {
   PreloadAction nextAction = PRELOAD_UNDEFINED;
   // If autoplay is set, or we're playing, we should always preload data,
   // as we'll need it to play.
   if ((AutoplayPolicy::IsAllowedToPlay(*this) == nsIAutoplay::ALLOWED &&
        HasAttr(kNameSpaceID_None, nsGkAtoms::autoplay)) ||

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -636,16 +636,22 @@ public:
     return mIsCasting;
   }
 
   void SetMozIsCasting(bool aShow)
   {
     mIsCasting = aShow;
   }
 
+  // Returns whether a call to Play() would be rejected with NotAllowedError.
+  // This assumes "worst case" for unknowns. So if prompting for permission is
+  // enabled and no permission is stored, this behaves as if the user would
+  // opt to block.
+  bool AllowedToPlay() const;
+
   already_AddRefed<MediaSource> GetMozMediaSourceObject() const;
   // Returns a string describing the state of the media player internal
   // data. Used for debugging purposes.
   void GetMozDebugReaderData(nsAString& aString);
 
   // Returns a promise which will be resolved after collecting debugging
   // data from decoder/reader/MDSM. Used for debugging purposes.
   already_AddRefed<Promise> MozRequestDebugInfo(ErrorResult& aRv);

--- a/dom/media/test/AutoplayTestUtils.js
+++ b/dom/media/test/AutoplayTestUtils.js
@@ -1,22 +1,22 @@
 function playAndPostResult(muted, parent_window) {
   let element = document.createElement("video");
   element.preload = "auto";
   element.muted = muted;
   element.src = "short.mp4";
   element.id = "video";
   document.body.appendChild(element);
-
+  let allowedToPlay = element.allowedToPlay;
   element.play().then(
       () => {
-        parent_window.postMessage({played: true}, "*");
+        parent_window.postMessage({played: true, allowedToPlay}, "*");
       },
       () => {
-        parent_window.postMessage({played: false}, "*");
+        parent_window.postMessage({played: false, allowedToPlay}, "*");
       }
     );
 }
 
 function nextWindowMessage() {
   return nextEvent(window, "message");
 }
 

--- a/dom/media/test/test_autoplay_policy_activation.html
+++ b/dom/media/test/test_autoplay_policy_activation.html
@@ -138,17 +138,18 @@
         async function runTest() {
           for (test_case of test_cases) {
             // Run each test in a new window, to ensure its user gesture
             // activation state isn't tainted by preceeding tests.
             let child = window.open(child_url, "", "width=500,height=500");
             await once(child, "load");
             child.postMessage(test_case, window.origin);
             let result = await nextWindowMessage();
-            SimpleTest.is(result.data.played, test_case.should_play, test_case.name);
+            SimpleTest.is(result.data.allowedToPlay, test_case.should_play, "allowed - " + test_case.name);
+            SimpleTest.is(result.data.played, test_case.should_play, "played - " + test_case.name);
             child.close();
           }
           SimpleTest.finish();
         }
 
         SimpleTest.waitForExplicitFinish();
 
       </script>

--- a/dom/media/test/test_autoplay_policy_permission.html
+++ b/dom/media/test/test_autoplay_policy_permission.html
@@ -29,17 +29,18 @@
           // Run test in a new window, to ensure its user gesture
           // activation state isn't tainted by preceeding tests.
           let url = testCase.origin + "/tests/dom/media/test/file_autoplay_policy_activation_frame.html";
           let child = window.open(url, "", "width=500,height=500");
           is((await nextWindowMessage()).data, "ready", "Expected a 'ready' message");
           child.postMessage("play-audible", testCase.origin);
           // Wait for the window to tell us whether it could play video.
           let result = await nextWindowMessage();
-          is(result.data.played, testCase.shouldPlay, testCase.message);
+          is(result.data.allowedToPlay, testCase.shouldPlay, "allowedToPlay - " + testCase.message);
+          is(result.data.played, testCase.shouldPlay, "played - " + testCase.message);
           child.close();
         }
 
         async function runTest() {
           // First verify that we can't play in a document unwhitelisted.
           is(window.origin, "http://mochi.test:8888", "Origin should be as we assume, otherwise the rest of the test is bogus!");
 
           await testPlayInOrigin({

--- a/dom/webidl/HTMLMediaElement.webidl
+++ b/dom/webidl/HTMLMediaElement.webidl
@@ -214,8 +214,18 @@ partial interface HTMLMediaElement {
  */
 partial interface HTMLMediaElement {
   [Pref="media.test.video-suspend"]
   void setVisible(boolean aVisible);
 
   [Pref="media.test.video-suspend"]
   boolean hasSuspendTaint();
 };
+
+/*
+ * API that exposes whether a call to HTMLMediaElement.play() would be
+ * blocked by autoplay policies; whether the promise returned by play()
+ * would be rejected with NotAllowedError.
+ */
+partial interface HTMLMediaElement {
+  [Pref="media.allowed-to-play.enabled"]
+  readonly attribute boolean allowedToPlay;
+};

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -584,16 +584,24 @@ pref("media.autoplay.allow-muted", true)
 // If "media.autoplay.default" is not ALLOWED, and this pref is true,
 // then audible media would only be allowed to autoplay after website has
 // been activated by specific user gestures, but non-audible
 // media won't be restricted.
 #ifdef NIGHTLY_BUILD
 pref("media.autoplay.enabled.user-gestures-needed", false);
 #endif
 
+// HTMLMediaElement.allowedToPlay should be exposed to web content when
+// block autoplay rides the trains to release. Until then, Nightly only.
+#ifdef NIGHTLY_BUILD
+pref("media.allowed-to-play.enabled", true);
+#else
+pref("media.allowed-to-play.enabled", false);
+#endif
+
 // The default number of decoded video frames that are enqueued in
 // MediaDecoderReader's mVideoQueue.
 pref("media.video-queue.default-size", 10);
 
 // The maximum number of queued frames to send to the compositor.
 // By default, send all of them.
 pref("media.video-queue.send-to-compositor-size", 9999);
 

--- a/testing/profiles/common/user.js
+++ b/testing/profiles/common/user.js
@@ -41,8 +41,9 @@ user_pref("security.turn_off_all_securit
 user_pref("xpinstall.signatures.required", false);
 // Prevent Remote Settings to issue non local connections.
 user_pref("services.settings.server", "http://localhost/remote-settings-dummy/v1");
 // Ensure autoplay is enabled for all platforms.
 user_pref("media.autoplay.default", 0); // 0=Allowed, 1=Blocked, 2=Prompt
 user_pref("media.autoplay.enabled.user-gestures-needed", true);
 user_pref("media.autoplay.ask-permission", false);
 user_pref("media.autoplay.block-webaudio", false);
+user_pref("media.allowed-to-play.enabled", true);