In bug 1469200, I found that Twitch's embedded player is calling a video's play() function not directly as part of the `click` event, but in a promise that is made while handling the click event: >key: 'play', >value: function () { > var e = this; > return this._srcReady.then(function () { > return new Promise(function (t) { > if (e._video.readyState >= 3) > t(e._video.play()); > else { > var n = function n() { > e._video.removeEventListener(o.c, n), > t(e._video.play()) > }; > e._video.addEventListener(o.c, n) > } > }) > }).catch (function (t) { > 'NotAllowedError' === t.name && (e._video.dispatchEvent(new CustomEvent(o.m)), e._video.dispatchEvent(new CustomEvent(o.k))) > }) >} Chrome allows the play promise to resolve, Firefox does not (it never resolves or rejects it). If I do not wait for the _srcReady or other promises, then it plays fine in Firefox (but is missing Twitch's error-handling): >key: 'play', >value: function () { > var e = this; > return e._video.play(); >} This makes me suspect that we should carry over permission to call play through into promise callbacks (and possibly requestAnimationFrame as well).

(Promises have nothing to do with event handling)