I disagree. Perhaps "NSFW" is too localized to actually be the name of the tag, but I think there should be a tag that means "give a warning before displaying this content." It would cover NSFW stuff, but could also work for spoilers, or answers to a quiz, etc. There's no way there could be culpability for omitting this sort of tag since it IS a value judgment, just like <em>.
Oh, but it absolutely should contain value judgements. There is nothing more semantic than value judgements, in fact. It's a tag that says "this content may be of questionable taste". How is that less semantic than <em>, saying "this content should be emphasized"?
Put this into a standard and you'll end up with a giant crazy mess. I can just imagine the idiotic lawsuits now.
Example. Some doofus somewhere looks at naked people on the web, on company time, against company policy and gets fired. Realizes that the webmaster didn't put up NSFW tags and sues the webmaster, the hosting provider and probably the upstream ISP as well. Before you know it we have yet another pointless, fruitless and unconstructive series of internationally televised debates on personal responsibility, provider accountability, etc. And because there's usually a bit of flesh involved somewhere, two thirds of the population will happily knee-jerk their way against common sense.
Yeah, brilliant idea this is.