Here are some useful resources for who is trying to learn or improve your HTML, CSS and JavaScript skills:
Dive Into HTML5
An amazing book about HTML5 for free, can you believe it? Go right now to diveintohtml5.org to learn about the HTML history, new HTML5 tags and much more.
WHATWG Community
The WHATWG group is the one who is creating the HTML5 specs. There’s a lot of useful information about HTML5 specs, a blog that you definitely should read, demos, etc.