In January 2010, YouTube launched an experimental version of the site that used the built-in multimedia capabilities of web browsers supporting the HTML5 standard. This allowed videos to be viewed without requiring Adobe Flash Player or any other plug-in to be installed. The YouTube site had a page that allowed supported browsers to opt into the HTML5 trial. Only browsers that supported HTML5 Video using the H.264 or <answer>WebM</answer> formats could play the videos, and not all videos on the site were available.
What format besides H.264 was playable with HTML5?
WebM