Fix Your Site With the Right DOCTYPE
http://alistapart.com/articles/doctype/

Zeldman, Jeffrey
List Apart, A
2002

Abstract:

Per HTML and XHTML standards, a DOCTYPE (short for “document type declaration”) informs the validator which version of (X)HTML you’re using, and must appear at the very top of every web page. DOCTYPEs are a key component of compliant web pages: your markup and CSS won’t validate without them.