Be liberal on what you accept...

In stackoverflow podcast #36, Joel & Eric are waxing eloquently about how adhering to the principle

Be liberal on what you accept and conservative in what you send.

causes massives problems for Web browser developers. While I understand that having a strict HTML / Javascript parser would ease our pain as engineers tremendously, I would argue that this very principle actually caused the explosive growth of the web in the first place. It opened up building web pages to thousands, if not millions of people whose brains aren't wired in the strange ways tradional programmers' brains are wired. A strict HTML parser spitting out cryptic error messages (and that's all you get from a strict compiler, trust me, I've seen them all) would have stopped them dead in the tracks.

Jeff Atwood has a slightly different, but interesting angle on the topic.

Just to beat an already dead horse into the ground... :-)