hero attribute on video players enables the following UX behavior:

• Video is moved to the top of the page with full bleed.
• Video autoplays
• Docking UX for the playing video Docking UX for video players #8088
• Maybe preload hero videos as part of preconnect (if not too heavy)
  hero attribute should be implemented as mostly a syntax sugar that brings existing independent effects.
  We already support autoplay.