Google has been a long time supporter of HTML5. They recently launched a HTML5 beta of YouTube however it will only work in Safari and Chrome. The reason for this is not due to the actual markup but the video codec chosen. YouTube is using h.264, the same codec used for YouTube HD via Flash. This works in Safari and Chrome because Safari uses QuickTime to render
<video/> and Google licensed h.264 for Chrome. Firefox however doesn’t include the proprietary codec for licensing reasons. It’s not a matter of cost but principle.
IE is supported through “Chrome Frame” which is essentially the Chrome browser in IE’s chrome. Your really just browsing the YouTube site with Chrome. Google could use this as a way to get people away from Flash and IE and onto Chrome one way or another.
I discussed the h.264 debate in more depth a few months ago.
You have to wonder why we don’t want anything proprietary slipping into HTML5, or want proprietary image formats (GIF turned us off to that) but exceptions are made for video.
Edit 1/23/2010: More on the topic:
Edit 5/21/2010: Thoughts on WebM.