DOCTYPE is the line that defines which version of the HTML spec your page is supposed to conform to. No HTML document can be valid without it. Which means that the vast majority of the HTML pages out there are invalid as most browsers don't insist on seeing it.

As for why you're having problems, I'm sure it's nothing to do with the DOCTYPE declaration. Can you post a cut-down version of the code that demonstrates the the problem.

"Perl makes the fun jobs fun
and the boring jobs bearable" - me