Autoplay of HTMLMediaElements

Autoplay is the concept of playing media elements without user gesture. The policy that defines when autoplay is allowed is defined here. The autoplay logic follows the HTML spec.

There are two ways of initiating autoplay:

• Autoplay by attribute: Setting the autoplay attribute on the media element. The element will try to autoplay when the readyState changes to HAVE_ENOUGH_DATA.
• Autoplay by play() method: Explicitly calling the play() method without user gesture.

All the autoplay logic is handled by the AutoplayPolicy class. When the media element wants to perform some action (like unmute, autoplay by attribute or play() method), it will send a request to AutoplayPolicy, and if the request is approved, the element can autoplay, otherwise it should be paused. Also the media element should inform the AutoplayPolicy about relevant changes such as “the element has been moved to a new document”.

User gesture lock

Each media element has a user gesture lock. If the element is allowed to autoplay, the lock is initialized as false, otherwise it's true.

When the element is trying to initate autoplay, we check the gesture lock. If the lock is false, it will be allowed. Otherwise autoplay will be blocked. An exception is that if the element is a muted video element, the gesture lock check will be bypassed.

To unlock the gesture lock (make it false). The page needs to call play() or load() on the element when responding to a user gesture.

Autoplay flowchart

The treatments of autoplay by different methods are different. For autoplay by attribute, it is:

This means if autoplay is initiated by attribute, it enters the autoplaying phase, we play it as long as the video is visible. When the page explicitly calls play(), pause(), we leave the autoplaying phase. When the page tries to unmute the video, we check the gesture lock and pause the video if it is still true.

For autoplay by play() method, it is:

This means if autoplay is initiated by play() method, we continue playing the video as normal play(). However if the page tries to unmute the video, we check the gesture lock and pause the video if it is still true.