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.
For an interactive view of features and which browsers they work in, see http://a.deveria.com/caniuse/
- canvas with some limitation implemented in Firefox, Opera, Safari, and Konqueror 4, with ExplorerCanvas emulated in Internet Explorer
- getElementsByClassName implemented in Firefox 3, Safari 3.1, Opera 9.5
- localStorage and sessionStorage implemented in Firefox 2, IE 8, nightly WebKit
- Database storageimplemented in Safari
- Custom content handlers partially implemented in Firefox 2
- ping attribute implemented in Firefox 3 (turned off by default) - note the implementation does not reflect the latest changes in the specification
- cross-document messaging implemented in Opera 9, Safari and Firefox 3, IE 8
- Audio interface implemented in Opera 9, Konqueror 4, Safari 3.1
- video element implemented in Safari 3.1 and Firefox 3.1 alfa 2, basic implementation in experimental Opera 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, Safari and Firefox nightly builds.
- event DOMContentLoaded implemented in Firefox, Safari, Opera
- Web Workers partly implemented in Firefox 3.1 alfa 2
- Offline Web applications implemented in Chrome (via Gears plugin), latest Safari on iPhone and Firefox 3.1
- inline MathML implemented in Firefox, partially in Opera (xhtml5 serialization only)
- inline SVG partially implemented in Firefox, Safari, Opera (xhtml5 serialization only)
Web Forms 2.0
- Opera 9 fully implemented Web Forms 2.
- Firefox 2 implemented input event (there is still bug #195696)
- Project webforms2 emulates a big part of WF2 in every browser
- date input element with input-type-date works in Firefox, IE6/7, Safari
- Repetition Model (dropped from the spec) - Opera 9 (native), with repetitionmodel works in Firefox, IE6/7, Safari 2.0.4
- Project  emulate in IE6/7 validation, date and time types, range, repetition, calculated fields, datalist, autofocus.
No implementations yet but there is work ongoing to implement this for Firefox.