Update June 3, 2012. Update to the latest CSS 3 schema even if you don't have Web Standards Update.
Web Standards Update provides the much wanted HTML5 & CSS3 support to Visual Studio 2010 SP1. It brings VS 2010 intellisense & validation as close to W3C specification as we could get via means of an extension. The most notable supported features by this extension are:
HTML5 – Video, Audio, Input Type, Drag & Drop, WAI-ARIA, Microdata, Schema.org
Note: This extension is created by a bunch of folks within Microsoft in their spare time. This is NOT an official Microsoft product