The Safari team:
Safari in macOS High Sierra uses an automatic inference engine to block media elements with sound from auto-playing by default on most websites. Safari 11 also gives users control over which websites are allowed to auto-play video and audio by opening Safari’s new “Websites” preferences pane, or through the “Settings for This Website…” option in the Safari menu. Further, a new power-saving feature prevents silent videos from auto-playing when either hidden in a background tab or otherwise off-screen.
Calling .play() on a <video> element will now return a Promise:
var promise = document.querySelector('video').play();
if (promise !== undefined) {
promise.catch(error => {
// Auto-play was prevented
// Show a UI element to let the user manually start playback
}).then(() => {
// Auto-play started
});
}