Back in January this year, YouTube launched a player that support HTML5 video tag. To try it out, you need to go to youtube.com/html5 and enable the player. What if, you want to switch to HTML5 player for a single video or you want to link to HTML5 version of the video? YouTube has a special parameter that lets you do just that: simply append "&html5=True" to video's URL: http://www.youtube.com/watch?v=cTl3U6aSd2w&html5=True
YouTube's HTML5 player is supported in Google Chrome (h.264; Chrome 6 adds support for WebM), Opera 10.6+ (WebM), Safari 4+ (h.264) and Firefox 4 (WebM).