Why can't every browser just interpret shit the same way, how hard is it actually to standardize the rendering?

Actually, standard is more or less here, it's Apple/Microsoft/Mozilla/Google/etc who don't respect it and push their approaches (and we gotta admit that it's getting much better with HTML5)

Because standards are just words, and words are made to be misinterpreted by different people.
Okay, I'm ignoring WebIDL here, but the basic point stands.

For some reason I've got the feeling that cs people are highly opposed to the idea of standards. In chemistry there was the sacred law of "IUPAC is right". It doesn't matter if this name implies a structure that isn't given, if IUPAC says that's its name that is the name. In cs it's like "W3C says this is the way to do this, so I'll contemplate it and will do it if it's not too much work". I'd have never thought I'd say this but I miss the dictatorship of a single standard defining entity...