YouTube HTML5 Video Player is an opt-in experiment for HTML5 support on YouTube. If you are using a supported browser, you can choose to use the HTML5 player instead of the Flash player for most videos. Your web-browser should support both the video tag in HTML5 and the h.264 video codec.

Supported Web-Browsers

• Google Chrome
• Apple Safari (version 4+)
• Microsoft Internet Explorer with Google Chrome Frame installed

Additional Restrictions:

• Videos with ads are not supported (they will play in the Flash player)
• If you've opted in to other testtube experiments, you may not get the HTML5 player (Feather is supported, though)

To join the HTML5 experiment, navigate to http://www.youtube.com/html5

See also:
How can I download HTML5 videos from YouTube?