I agree with you, that this is great for writers & readers of sites that already mark their content as not safe for work.
However, I haven't seen anyone point out this problem, which lends itself to other complaints slashdotters have had: Currently, some page writers put "NSFW" next to objectionable links. If NSFW moves to an HTML relationship value, then you won't have a visual cue that the author marks stuff as NSFW or not.
Even if the browser alerts you when you mouse-over an objectionable link, when you mouse-over a non-NSFW link, you won't see a cue to tell you if it's safe or if the author failed to mark it as NSFW. If you're at work and worried about these things, then you'll have to do the same as you do now: judge based on the rep of the site and such, and assume that any unmarked link might lead to racy content.
If you're forced to make that assumption, perhaps you should make it official. Make an HTML relationship value "ISFW" (Is Safe For Work). Then readers can assume that unmarked links are dangerous, that marked links are safe according to the author. The browser can report that the link is marked safe (say, through the pointer icon or through a tooltip).
Just a proposal, but I think it's important to point out that an absence of NSFW tags doesn't tell you if they've been used or not. You lose the visual info that you occasionally get now.