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The strange thing is that the page works fine if we remove it. I'll look at the CSS again but it seems to me there should be no relationship between the css and doctype. But then I'm kind of clueless about this.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> The difference between the doctypes is that the "full" version ensures "standards-compliance mode" in modern browsers, where the specifications are much more closely-followed. The shorter doctype (without the URL) triggers a special, more backwards-compatible "quirks mode". See tedster's excellent library thread Quirks Mode vs. Standards Mode [webmasterworld.com] for more information.
Ideally, you should move to using standards-compliance mode for all documents.