Using DocType properly

Fixing Your Site with the Right DocType

"You've done all the right things, but your site doesn’t look or work as it should in the latest browsers.

You’ve written valid XHTML and CSS. You’ve used the W3C standard Document Object Model (DOM) to manipulate dynamic page elements. Yet, in browsers designed to support these very standards, your site is failing. A faulty DOCTYPE is likely to blame.

This little article will provide you with DOCTYPEs that work, and explain the practical, real–world effect of these seemingly abstract tags." [A List Apart]