and why should browsers even supply codecs? why not just use whatever framework is available from the OS. IIRC, the gtk webkit port uses gstreamer for video support, so it probably can handle quite a few codecs apart from theora and h264

They can just as well ignore official support, and just provide binaries (and _maybe_ answer some questions in the forums if they don't happen to run on a particular system). They don't have to support anything, not even Ubuntu. What's the worst that can happen if they release unsupported binaries? Lose users? Linux users already aren't able to play their games anyway.