YouTube's HTML5 support is offered through a beta opt-in program and is currently only available with browsers that support both HTML5 and H.264 video encoding.
This limits the beta to Google's Chrome and Apple's Safari browsers. Microsoft's Internet Explorer is also supported if Google's Chrome Frame is installed.
The HTML5 player fully replaces the Flash player for most videos on the site, but some limitations do exist. Videos with advertisements are currently not supported, and will play instead in the Flash player. Fullscreen playback is also not yet supported.
YouTube refers to the opt-in as "an experiment" and welcomes feedback from the community on its HTML5 integration.
HTML5 allows video to be rendered natively inside the browser, as opposed to Flash which requires a browser plug-in to be installed.
Flash is still not supported on Apple's iPhone. Major sites that use Flash for video on the web, including YouTube, CBS Mobile, and the BBC, now push standard H.264 video to the iPhone directly.
Beginning Thursday, video site Vimeo also will begin to support HTML5 playback. Playback using the HTML 5 standard is available on the same browsers that YouTube supports: Chrome, Safari, and IE with Chrome Frame.
For more, read AppleInsider's Flash Wars series:
Adobe in the History and Future of Flash [Part 1 of 3]
The Many Enemies and Obstacles of Flash [Part 2 of 3]
Adobe Fights for AIR with the Open Screen Project [Part 3 of 3]