WPT details: The reflection is tested here: https://github.com/web-platform-tests/wpt/blob/master/interfaces/html.idl#L420 The actual behaviour is left to the UA and can't really be tested at the moment.
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.