We already have a rift when it comes to the VIDEO tag.
Gecko browsers support Ogg codecs, while WebKit browsers support H.264. Each group does not support the others' codecs.
It looks like H.264 is going to win primarily on the strength of Apple and Google (Safari and Chrome). It's also rumored that Microsoft/IE will be joining the H.264 camp, but I'm sure they'll dick it up by supporting WMV as well, and some dumbass site owners will use it, ensuring that there are STILL IE-only, Windows-only websites out there. (unless the other browsers tap into Flip4Mac on the Mac and WMP ActiveX on Windows - and then that still doesn't support all the cool flip effects and stuff).