HTML5 is not a video codec. The HTML5 video tag will work with different codecs... depending upon the browser.
IE9 supports MP4 (H.264video and AAC audio)
Firefox 4 supports WEBM (vp8 video and Vorbis audio) and OGG (Theora video, Vorbis audio)
Google Chrome supports MP4, WEBM, and OGG
Safari supports MP4
Notice the issue... If you encode as mp4 it won't run on Firefox. If you encode as WEBM it won't run on Safari. For full coverage using html5 you need to do something like
this [w3schools.com]. At least for the short term. Mozilla recently decided that they will support H.264 video.
Anything that can output H.264/AAC in an mp4 container will meet most of your needs.