YouTube has updated the mobile version of their site with several important improvements, including now serving videos using the HTML5
video tag. If you’re using an HTML5 compliant mobile browser, which includes the iPhone and Android phones, you’ll get the new and improved site. If you hit m.youtube.com with an older phone (or a desktop web browser) you’ll get the older, more basic version instead.
The redesigned mobile site features touch-friendly interface changes, speed improvements, and much higher quality video than the previous Edge-optimized version. The original mobile site launched in 2007 and the native iPhone YouTube app has hardly been updated since the first iPhone was released, so many of the changes revolve around getting the mobile site on the same page as changes to the full version of the site. Most importantly, users can now choose to watch the high quality versions of videos when available.