Web site validation is an issue that will become increasingly important with the emergence of newer, even more standards-compliant browsers. Browsers which, for the first time, will treat with contempt – possibly even reject – ill-formed websites. Upcoming versions of both Firefox and Explorer promise rigorous standards-compliant rendering.
Are you up to the challenge? Gareth Powell explores the fundamental notions on web site validation, DOCTYPE validation and its consequences in the white paper: "Web Site Validation: The Fundamentals". Check it out, it is worth it!