I'm trying to get my site W3 validated, but one thing I noticed is when I add the doctype tag, the whole design goes to hell. Why is this happening?
Because you coded the site in (shit-mode) excuse me quirks mode. To get consistency in all browsers you NEED to use a DTD such as HTML 4.01 Strict or XHTML 1.0 Transitional.
Be sure you're using the right doctype. Here's an article: alistapart.com/stories/doctype/ Also be sure your web site validates: validator.w3c.org
First off, you were designing your site in quirks mode, meaning that the browsers had to guess how you wanted the page to be presented to the user. BAD IDEA. Second, I'm going to presume that you were designing for a browser, rather than to the specifications. I prefer a browser-agnostic approach, testing each section of the Web page in Internet Explorer 6/7, Firefox, Opera and Safari as I go along, rather than waiting until I'm done to test. Doing so allows me to catch a lot of errors that would be difficult to fix without resorting to hacks if I were to wait until the end and/or code to one browser and hack for the others.