H88: Using HTML according to spec
Of course we cannot have an Ultimate SEO Guide without reference to the HTML Specifications themselves. They are the foundation of all things you will do as an SEO. Since SEO involves everything to do with the document itself, it is only natural that you have a basic understanding of what is under the house. It would be difficult for you to make proper structural recommendations if you had no idea how the foundation was constructed.
One thing I'd like to point out when reading the W3, any time you see that phrase User Agent utilized, pay close attention. There is "usually" a direct correlation with that phrase and the search engines to some degree. You'll find that phrase used extensively throughout the HTML specifications.
Have you noticed that the WCAG 2.0 Documents provide you with working examples of scripts and such? That is way cool. I like cutting, pasting, and having things work the way they should. :)
Note: I did NOT mention the words HTML/XHTML Validation above.