Without a doctype you can't
validate the document. Even if you don't care about standards, validation is a quick way to find mistakes that may (or may not) lead to problems.
Since some years now browsers have two rendering modes, Standards Mode (AKA Strict Mode) and Quirks Mode. What mode they'll use is triggered by the doctype. No doctype at all will guarantee you Quirks Mode. In that mode browsers emulate older versions of themselves and in same cases older versions of IE (Opera does this). Standards Mode will iron out many (but far from all!) inconsistences between browsers. Either way you go, it's better to choose your rendering mode yourself.
Note that a Strict doctype doesn't necessarily give you Standards Mode and a Transitional Doctype doesn't necessarily give you Quirks Mode. It just so happened that the first browser that did this (IE5 Mac) used the doctype as a trigger and the rest of them of course followed suite.
http://www.hut.fi/u/hsivonen/doctype.html
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http://yupapa.com
Why to use <!Doctype...?
