Choosing the Right Doctype for Your HTML Documents
http://dev.opera.com/articles/view/14-choosing-the-right-doctype-for-your/

Johansson, Roger
Opera
2008

Abstract:

In this article I will look at the doctype in a lot more detail, showing what it does and how it helps you validate your HTML, how to choose a doctype for your document, and the XML declaration, which you’ll rarely need, but will sometimes come across.