|Browser||Can play Ogg Theora||Can play H.264||Can play WebM|
That last row may come as a surprise if you've not paid attention to how Microsoft have implemented the video tag and just read all the FOSS propaganda. Microsoft have used the existing DirectShow framework, and as a result IE should be able to handle anything that there's a DirectShow codec for. And there's DirectShow codecs for pretty much any format in existence, including Theora and WebM.
It amuses me no end that the browser usually ridiculed for poor compatibility has the best HTML5 video support.
Edit: corrected table headings.