YES, I've been thinking about this kind of thing for a while. But <nsfw> doesn't seem like enough.
What if an entire page or website is nsfw? Maybe some kind of header information would be more appropriate. Or inside the root element (<html nsfw>). How about specific ratings? <nsfw rated:"urn:mpaa:R">
There are still plenty of problems--kids getting around the browser settings--but it's 10x stronger[citation needed] than "what's your birth date?".