Many people make spelling mistakes, so does that make dictionaries totally pointless too? As has been discussed at length (eg.
here and
here) errors can have a considerable detrimental effect. Even a simple (petty) error can cause blind people and search engines to miss content even if it renders in the regular browsers. There is, of course no compulsion to follow the recommendations (or spell correctly) but generally it's good practice and if you do happen across a browser oddity it's going to be a lot easier to discover the cause with clean code.