Making a full page background video is slightly trickier than a full page background image. Over on the Media Temple blog, I take a look at how that’s done, but then also what the design patterns are once you’ve done it. You likely need text over top the video, so do you center it? Do you let the page scroll and cover the video? Do you get fancy and fade out the header as you scroll?
Source: CSS Tricks Full Page Background Video Styles