Implementations in Web browsers
List of known implementations of HTML 5 in web browsers (list is incomplete, feel free to extend it)
For summary see Comparison of layout engines (WHATWG) in Wikipedia.
- canvas with some limitation implemented in Firefox, Opera and Safari, with ExplorerCanvas emulated in Internet Explorer 6/7
- getElementsByClassName implemented in Firefox 3
- Client-side session and persistent storage implemented in Firefox 2
- Custom content handlers partially implemented in Firefox 2
- ping attribute implemented in Firefox 3
- Cross-document messaging implemented in Opera 9
- Audio interface implemented in Opera 9
- video element basic implementation in experimental Opera build and experimental Firefox build; with mv_embed script in every browser with Java support
- Server-sent DOM events implemented in Opera 9
- contenteditable attribute implemented by Opera, Internet Explorer, Safari and Firefox 3
- Drag & drop (partially) implemented in Internet Explorer and Safari.
Web Forms 2.0
- Opera 9 fully implements Web Forms 2.
- Project  emulate in IE6/7 validation, date and time types, range, repetition, calculated fields, datalist, autofocus.
Web Controls 1.0