You Too Can Experience HTML5 on YouTube
HTML5's support for audio and video promises to be a more efficient way to watch movies on your PC through your browser rather than the Flash player (although the beta of Flash Player 10.1 improves HD video playback on some PCs). You can sign up for the beta simply by visiting YouTube's HTML5 page and start enjoying HTML5 videos today, but there are restrictions: videos with captions, annotations, or ads will still be rendered by the Flash player, and fullscreen mode isn't supported.
Despite Firefox's and Opera's support for HTML5 standards, YouTube's experiment will only work with Safari 4, Chrome, and Internet Explorer via ChromeFrame. If you're unsure if your browser will work with HTML5, check out YouTube's TestTube to verify compatibility.