Implementations in Web browsers
From WHATWG Wiki
This is a list of known implementations of HTML5 and related features in web browsers. (It is incomplete, feel free to fix it.)
- 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, Safari 4
- Database storageimplemented in Safari 3.1 and iPhone Safari 2.0
- 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), Safari 4, iPhone Safari 2.1 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)
- List of future Firefox implementation tracking bugs
- 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.
- Chrome dev channel (126.96.36.199 or later) implemented WebSocket.
- There is work ongoing to implement this for Firefox and Safari/WebKit.
- pywebsocket implemented server side.
- GNU WebSocket4J, a WebSocket protocol implementation in Java.