gstreamer is quite mature and can be used on Windows and Mac OS X as well. So there are no reasons to make different backends for each OS.

Mozilla is going to implement gstreamer backend for html5 video element. See Bug 422540.

Also, Opera developers are going the same way. See this blog post.

Using gstreamer as a backend will eliminate ALL the problems with codecs. Forever. It will be able to play just the same as a usual desktop players, and that means, it will be able to play Ogg Theora, H.264, DivX, whatever you like - it's only a matter of plugins installed.

Sounds great, but the situation is that anime cannot be watched with the default Ubuntu player. It lacks support for ASS subtitles and MKV linked chapters. The only good approach to this problem now (for any distribution) is to build mplayer from git development tree (for MKV linked chapters).