Umm, 20 and 50 what? You forgot to specify a unit of measurement
That's where browsers are going to choke. When you don't specify a unit of measurement, the browser will either ignore the rule, or attempt a guess at what you meant. By removing the loose part of the doctype, you throw IE into Quirks mode. In Quirks mode, IE will have a field day trying to guess at what you mean. In Standards mode, IE will show a little more restraint. I assume you were seeing this behavior in IE right? Removal of the second part of the doctype doesn't cause Moz/Opera to revert to Quirks.