Ok, I got it working in every common browser, with a fallback for the olders ones. I used VLC to convert the files, and used the HTML below:
<video id="video_background" volume="0" muted="muted" loop="loop" autoplay="true" preload="auto">
<source type="video/mp4; codecs=avc1.42E01E,mp4a.40.2" src="video/pr6.mp4"></source>
<source type="video/webm; codecs=vp8,vorbis" src="video/pr6.webm"></source>
<source type="video/ogg; codecs=theora,vorbis" src="video/pr6.ogv"></source>
<img alt="bg" src="img/bg1.jpg">
</video>