This is a hint a website may provide to a user agent to display a video content within the element's playback area.
It allows embedders to provide a media experience where a playing a video automatically enters fullscreen. This attribute will offer an opt-out for websites. Safari on iPhone already supports this.
Docs: Discussion to add it to spec: https://github.com/whatwg/html/issues/1237
Explainers: N/A