The W3C Site for further information - http://www.w3.org/
This is not intended as a replacement guide to the W3C's guidelines, but is merely presenting existing information in a slightly more condensed and accessible format.
It is always wise to check your HTML code with a code validator such as the one found at (http://validator.w3.org) and to run through the code with a code cleaner, such as the one found in ScriptWorx.
Ideally, documents should be validated up to Level 4.0 HTML, which can be defined using the !DOCTYPE tags discussed in the HTML reference.