HTML comes in many flavors, including the XHTML subsets. Each "type" of HTML includes various allowable elements and properties. A browser needs to know which type of HTML your document uses, thus, the "DOCTYPE". If no DOCTYPE is provided, the browser will fall into what is know as "quirks" mode, meaning, they will do whatever the designers of the browser set as the default for any given element. This causes the differences one sees between IE and FF. To prevent this, you must specify a proper, complete doctype. We can't tell you which you should use, because we don't know which HTML specification was used to code the page.

Referece: http://www.htmlhelp.com/reference/ht...l/doctype.html