IEBlog - HTML5 Video
blogs.msdn.com/b/ie/archive/2010/04/29/html5-video.aspx
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
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.