Check out the HTMLPedia, a user-maintained wiki for all you want to know, or what you what to “say” about the HTML / XHTML markup language.

The HTMLPedia is maintained by the same individual who brought you the Mozilla Addon, HTML [Tidy] Validator, a popular Firefox extension which comes in handy for web developers who wish to adhere to web standards by paying close attention to the structural aspects of the markup in their XML, XHTML, HTML, or other SGML-based source-code.

The HTML Validator extension parses a document when it is loaded into the browser (either a ‘local’ or remotely hosted document), checking for well-formed SGML, and / or XML markup. Thus, the Validator, not unlike a spell-checker, rapidly helps the developer to recognize where he or she may have authored improperly structured markup, by signifying errors in the document (where applicable, directly corresponding to the <!DOCTYPE> being used) through informative icons and/ or text in the browser status-bar.