Robert_Charlton - 7:33 pm on Apr 20, 2011 (gmt 0)

If you follow enough links, you'll get to...

http://www.webmproject.org/about/faq/ [webmproject.org]
...and, eventually, to...
How do I play WebM files?

To play WebM files, all you need is a supported web browser or media player [webmproject.org].

(Note that the hash mark in the "supported web browser" link above will break through WebmasterWorld's link redirect in some browsers, in which case you either need to scroll down the page or else paste the following link into your address bar....)

http://www.webmproject.org/users/#supported_web_browsers

Supported Web Browsers - Mozilla Firefox 4 and later - Opera 10.60 and later - Google Chrome 6 and later - Microsoft Internet Explorer 9 and later (requires WebM MF components)

From the parenthetical comment after IE9, it appears, since IE9 isn't available for XP, that WebM will require Windows 7 to play in Internet Explorer. As the blog article suggests, it will be a gradual migration, and this is YouTube taking "an important first step".