Safari uses Quicktime for their HTML 5 video. Any Quicktime codec works. For example you can install XiphQT and get Theora support.
Opera (on Linux only) supports any GSteamer installed codec IIRC.
Is quicktime on Mac better than Windows? On windows it's a plugin and really really bad.
There's a QuickTime plugin, but it's also a set of multimedia APIs, and Safari uses those directly to implement HTML5 video. It works pretty well - one of the nicest-looking and most CPU-efficient video experiences you can get in a browser.
sadly Quicktime is broken beyond repair, i.e. you can not do HTTP range requests to allow seeking in videos beyond the part that is already downloaded. On top of that Apple knows about this problem but ignores it. They even went one step further and just fixed it for H.264 in Quicktime X. (note that they stopped supporting 3rd party components in Quicktime X)
IE9
IE9 does, but it only whitelists two codecs, H.264 and VP8/WebM. How this will actually work, and whether it will allow you to install alternative versions of either codec or whether only the native H.264 and a specific VP8 decoder are blessed, I'm not sure.