Background videos could be partially supported through two different techniques - an overlay layout with two assests, or a HTML with Video asset

'Overlay' Layout
HTML with video
<video> Element
Animated gif
Not supported

Works with major issues. 

Anything in your html which is not coloured or pure black, will be replaced with video. Unsure of threshold, #010101 will get replaced

Partially transparent parts of the html will not display the background video - instead it will blend it with pure black.

The video file will continue to replace black pixels in following html assets in the schedule

Not supported - shows poster image
Plays but is choppy
Works smoothly

Works with issues. 

On every second play through of a HTML with Video asset, it will just play the video, and not display the html.

RGBA will be first blended with black before being blended with the background. e.g. 50% white will come out as transparent grey.

Works perfectly
Plays smoothly

Not supported - will skip in schedule
Does not autoplay
Plays smoothly

The animated gif was tested with a 14mb 720p gif - to get a full background a much much larger gif must be used (at least 12x resolution). Doubt it would work.