As most of you will know by now, HTML5 includes the video and audio tags, which allow you to embed video and audio files in your HTML document as if they were images. All modern browsers support the HTML5 audio and video tags (except Internet Explorer), but sadly, that doesn't mean the new HTML5 YouTube beta will work on all of those browsers.
The problem is the codec. HTML5 doesn't specify the codec to be used with the video tag, leading to a situation where everybody's debating either Theora or h264. Theora supposedly isn't as good as h264 (note the supposedly, I hear conflicting statements on that one), but h264 is a licensing nightmare, so not all browsers support it. Chrome and Safari both do, but Firefox and Opera only support Theora. Since the original feature request stressed using open standards (i.e., Theora), Google still has some way to go.
YouTube using h264 is just one of the limitations of the HTML5 version of YouTube. It also won't play videos with annotations, advertisements, or captions; for those videos, it will switch back to the Flash player. This being in beta stage, these are acceptable shortcomings.
What I do not find acceptable, however, is the abysmal video quality of videos played using the HTML5 version. They are ridiculously blocky on my setup (Chrome 4.0 on Ubuntu 9.10), giving me the feeling I'm back in 1999. It's really quite startling to compare the Flash version of a video to the HTML5/h264 version (I suggest trying it out on this one - isn't it great I can use OSNews to subtly promote my favourite music?).
Of course, there is also a major, major advantage in using the HTML5 version: processor usage. Playing the A Night Like This video in Flash 10 on Ubuntu eats about 68% (!) of my quad-core processor, whereas the HTML5 version sticks at about 12-20%.
In order to join the beta, simply visit this page and click "Join the HTML5 beta"; you can click it again to leave the beta version.