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.
- Web DOM4 Core -- culling the server-side features from DOM3 Core, and adding features that would help the web.
- CSSOM - the specification for scripted manipulation of style, both the existing interfaces and new interfaces to make it easier.
- DOM3 Events
- Keyboard Events -- .charCode, .keyCode, and related stuff (mostly defining legacy interfaces)
- Bindings For DOM -- defining IDL