enum AutoplayPolicy {
  "allowed",
  "allowed-muted",
  "disallowed"
};

enum AutoplayPolicyMediaType { "mediaelement", "audiocontext" };

[Exposed=Window]
partial interface Navigator {
  AutoplayPolicy getAutoplayPolicy(AutoplayPolicyMediaType type);
  AutoplayPolicy getAutoplayPolicy(HTMLMediaElement element);
  AutoplayPolicy getAutoplayPolicy(AudioContext context);
};

if (navigator.getAutoplayPolicy("mediaelement") === "allowed") {
  // Create and play a new media element.
} else if (navigator.getAutoplayPolicy("mediaelement") === "allowed-muted") {
  // Create a new media element, and play it in muted.
} else {
  // Autoplay is disallowed, maybe show a poster instead.
}

if (navigator.getAutoplayPolicy("audiocontext") === "allowed") {
  let ac = new AudioContext();
  ac.onstatechange = function () {
    if (ac.state === "running") {
      // Start running audio app.
    }
  };
} else {
  // Audio context is not allowed to start. Display a bit of UI to ask
  // users to start the audio app. Audio starts via calling ac.resume()
  // from a handler, and 'onstatechange' allows knowing when the audio
  // stack is ready.
}

function handlePlaySucceeded() {
  // Update the control UI to playing.
}
function handlePlayFailed() {
  // Show a button to allow users to explicitly start the video and
  // display an image element as poster to replace the video.
}

let video = document.getElementById("video");
switch (navigator.getAutoplayPolicy(video)) {
  case "allowed":
    video.src = "video.webm";
    video.play().then(handlePlaySucceeded, handlePlayFailed);
    break;
  case "allowed-muted":
    video.src = "video.webm";
    video.muted = true;
    video.play().then(handlePlaySucceeded, handlePlayFailed);
    break;
  default:
    // Autoplay is not allowed, no need to download the resource.
    handlePlayFailed();
    break;
}

let ac = new AudioContext();
if (navigator.getAutoplayPolicy(ac) === "allowed") {
  ac.onstatechange = function () {
    if (ac.state === "running") {
      // Start running audio app.
    }
  };
} else {
  // Display a bit of UI to ask users to start the audio app.
  // Audio starts via calling ac.resume() from a handler, and
  // 'onstatechange' allows knowing when the audio stack is ready.
}