Grammar, punctuation and the likes.
Check this out. And this, esp. the part about quotes.
Dude, you can write <big><b>shit</big></b> and get a big bold shit, and it's valid HTML. But that's an exception and it's certainly not legal XML, and as a programmer I cannot write ' "Fuck," Bill said ' , because it's stupid. And as Wikipedia explains, it's an outdated tradition due to technical reasons having to do with typesetting, rather than some legitimate language rule. I wouldn't care anyway.