- Go to http://www.youtube.com/html5
- Click on “Join the HTML5 Trial“
Most Youtube videos you watch will be served in WebM format with HTML5 markup, instead of Flash.
Sidenote: Before you go thinking Firefox 4 is the first version of Firefox to support HTML5 video, it should be noted the HTML5 doesn’t specify video codec. Firefox has had support for HTML5 video since version 3.5.