When you didn't have a doctype, you didn't set the rules, and browsers do what they will while IE goes into 'quirks mode'. Now that you add a doctype, the rules are changed. So now the browser is following a different set of rules.
You don't necessarily need the specs for XHTML 1.0 because I doubt you are serving it, but
here they are anyway. Pay particular attention to the part about differences with HTML because you are actually serving XHTML as HTML.
A doctype is required of all web pages.