http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2009-June/020620.html
Unfortunately OGG Theora didn't make it as the default codec for the HTML5 <video> element in the spec. Until one of the two major formats (Theora and H264) is clearly the major format the HTML5 spec will not specify a default codec for the <video> element.
-- Hay