There are three video formats that work natively in some browsers. Unfortunately, no format works in all browsers, so you need to do at least two if you want meaningful HTML5 video support.
Minimally, you must use MP4 + H.264, with AAC or MP3. MP4 video plays natively in Safari, Chrome, and IE9 (Vista/Windows 7). It is also your best option for a Flash video fallback, and plays natively on many devices (iOS, Android, Blackberry, PSP, Xbox, PS3, etc.). Use H.264 High Profile for the best quality, or Baseline profile if you want the same video to be playable on mobile devices.
Beyond that, use either WebM + VP8 or Ogg + Theora with Vorbis audio for other browsers. WebM works in Firefox (4+), Chrome (6+ or Chromium), and Opera (10.60+), and Ogg works in Firefox (3.5+), Chrome (3+), and Opera (10.54+).
