HTML5 is a documentation spec, not a video encoder. I think you knew that, but language is surprisingly precise, so before commenting I wanted to ensure that we were talking about the same thing.

Having got that out of the way, the HTML5 spec for the VIDEO tag is a bit tenuous at the moment. Though the API is laid out in full, there are essentially three competing encodings vying for the default: Ogg/Vorbis, MPEG-4, and VP8. Only Mozilla supports Ogg as other parties feel uncertain about its defensibility in a patent war against MPEG-LA. MPEG-4 is not F/OSS, ergo Mozilla isn't supporting it. Google has purchased and opened VP8 in hopes that it will become the open standard, thereby assuaging Mozilla's and the F/OSS' community's fears, though Apple has yet to be sold on VP8's defensibility again MPEG-LA.

To wrap this up, if you do a little digging you'll find that Apple used the MOV container to hold an MPEG-4 video. This is just fine and dandy in the current state of the spec, though it would have been nice for Apple to use a different container format.