DOCTYPEs can really break your site. If they are wrong. Problem is, not many people know the correct DOCTYPEs. Hell, most editors put in the wrong DOCTYPE for you.
What’s a DOCTYPE? A DOCTYPE informs the validator which version of (x)HTML you’re using, and has to appear at the very top of every web page. Has to be there.
So, as with any required element it seems, the browsers implement them differently. Shocker.
Here are some sites that help out with the DOCTYPE issues: