Hexus reports YouTube is experimenting with a new HTML5-based video player, a feature that may eventually replace Adobe's Flash Player. You can give the HTML5 version of YouTube a try over here, but some browsers like Firefox don't support it yet.
At present, supporting browsers are few and far between. With the HTML5 standard yet to finalise specified audio and video codecs, browser compatibility remains hit or miss. YouTube's experimental player makes use of HTML5's video tag and the h.264 video codec - resulting in compatibility with just a handful of browsers; Chrome, Safari and ChromeFrame on Internet Explorer.
In addition to the lack of browser support, YouTube's HTML5 player doesn't yet support the playback of videos with ads, captions or annotations, and doesn't allow for full-screen viewing.