The ultimate resource for learning HTML5 is its specification. W3 is the authority behind the HTML5 specification. The HTML5 specification in the link is still not in its final version but this is the basis for HTML5, so dramatic changes are unlikely to happen in the final version. Related posts:

For a comprehensive explanation and reference of HTML5, visit W3’s site. The page is very detailed and in depth, but it also has a getting started section that will help you create a sample document. Related posts: HTML5 Specification (Editors’ Draft) HTML5 Cheat Sheet CSS Reference

W3’s link checker tool is very simple, but works well. There are lots of link checkers out there, but many have limits on how many pages they’ll check, or you have to create an account and log in to use it. Depending on how big your site is, this free