HTML5 doesn't stand alone, it's just one of many specifications that form part of the Web platform. Many of these are currently orphaned or neglected; if you want to help, you could try editing one!
- CSS3 - there's a whole slew of specifications for extending CSS that are in need of editors. In particular:
- the CSS3 Flexible Box module
- updates to DOM2 Traversal and DOM2 Range.
- CSSOM - the specification for scripted manipulation of style, both the existing interfaces and new interfaces to make it easier. (Parts are being done by Anne at the moment.)
These sections of HTML5 could easily be taken out and make into independent specs:
- 2d context for canvas (this is pretty stable right now, so it would probably be a bad time to work on it)
- 3d context for canvas
These specs currently have active editors, but have historically spent time abandoned:
- DOM3 Events
- Keyboard Events -- .charCode, .keyCode, and related stuff (mostly defining legacy interfaces)
- Web DOM4 Core -- culling the server-side features from DOM3 Core, adding features that would help the web, adding generic exceptions that would help other specs out.