Include a proper Doctype

You start your sentences with a capital letter; start your HTML with a DOCTYPE. It’s just basic grammar. – Mark Pilgrim from Dive Into Accessibility

Using a complete doctype will switch your browser into standards mode. This will cause it to render the page exactly as you have instructed it (which may look awful but at least now you can point the finger at yourself and not the browser). Without a proper doctype your browser will switch to quirks mode or as I refer to it “amateur mode”. A proper doctype with an XML prolog will also cause some browsers (IE) to switch into quirks mode so do not add an XML prolog.