Originally Posted by Duekster
most browsers will just ignore code it can not parse.
A.K.A quirks mode. IE 4 & 5 are notorious for being non-compliant, but you can still write valid code for them, even if you had to jump though some hoops to get some of the CSS 2 markups to work properly. Admittedly, some of those CSS workarounds made the CSS invalid, strictly speaking.