Not too long ago, Microsoft released an extension for Google Chrome which enabled H.264 HTML5 video playback. Now Google has returned the favor by offering a WebM plug-in for Internet Explorer 9 users. In a move which we can only describe as oozing with self-confidence, Google points out that there are some known issues -- visit this page, and revel in its blankness.
If you plan on using IE9 but don't want to miss out on all the WebM videos which are popping up, download and install the plug-in now. The plug-in only works on Vista and Windows 7, but conveniently the same is true for IE9.