HTML Coding Rules : Rule 1
DOCTYPE
< !DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Strict//EN"
"http://www.w3.org/TR/html4/strict.dtd">
The DOCTYPE is used to determine how the browser will render your code. There is one standard for HTML, but not one browser. Having the DOCTYPE in the beginning of all your HTML pages helps keep the browser defined in a specific set of standards.
HTMLTree comment: Most web programmers would argue there exist two sets of standards. Microsoft being the latter. But with MSIE 6 or later, Mircrosoft states it will follow w3 standards if the DOCTYPE exist.
via [HTMLTree]