I spent quite a lot of time trying to figure out why video embedded like here:
<video height="256" loop autoplay muted controls id="vid">
<source type="video/mp4" src="video_file.mp4"></source>
<source type="video/ogg" src="video_file.ogg"></source>
</video>
starts playing automatically once the page is loaded in FireFox but cannot do autoplay in Webkit based browsers. This only happened on some random pages. So far I was unable to find the cause. I suspect some unclosed tags or extensive JS created by CMS editors.