IE9 Will Support HTML5’s Video Element but Only With H.264 ↦

From the official IEBlog:

The future of the web is HTML5. Microsoft is deeply engaged in the HTML5 process with the W3C. HTML5 will be very important in advancing rich, interactive web applications and site design. The HTML5 specification describes video support without specifying a particular video format. We think H.264 is an excellent format. In its HTML5 support, IE9 will support playback of H.264 video only.