I don’t know much about videos but recently had to do a full screen video and this format worked well for me.
<video id="bgvid" loop autoplay preload="auto" poster="HP_002.jpg">
<source src="video/HP_002-HD_720p.mp4" type="video/mp4">
<source src="video/HP_720p.webm" type="video/webm">
<source src="video/HP_002-HD_720p.ogv" type="video/ogg">
</video>
As felgall said for modern browsers you may just need the 2 formats.