I'm not normally a reblogger, but is a "necessary share".
HTML5 Validation - Video & related tags, Audio & related tag, New input types like email, url, date etc, Drag and Drop support, Accessibility standard WAI, Accessibility standard WAI-ARIA, Microdata, Schema.org plus SEO
Browser APIs - Schema validation for GeoLocation and Local Storage
CSS3 - 2D Transforms, 3D Transforms, Animations, Background & Borders, Basic Box Model , Basic UI, Behavior, Color, Flexible Box Layout, Fonts, Generated Content for Paged Media, Hyperlink Presentation , Line, Lists, Marquee, Media Queries, Multi Column, Namespaces, Paged Media , Presentations Levels, Ruby, Selectors, Speech, Syntax, Template Layout, Text , Transitions
XBrowser - browser specific prefixes like –ms; also other vendor prefixes like –webkit and –moz .
You can read more on the webtools team blog. Visha Joshi @vishalrjoshi also wrote an excellent post detailing exactly what tags and features and such are supported in this release at http://vishaljoshi.blogspot.com/2011/06/announcing-html5-css3-support-for.html .