alt Hacker News> I imagine it could be not trivial to enforce (esp. for offshore web)
It's quite trivial, actually - the parental control software is designed so that if there are no content tags, then the site does not display. The mandate for websites to tag their content would only need to apply to websites over a certain size, to bootstrap the network effects.
The other option is for the major browsers to refuse to load pages that don't include the tag. I don't think it's a good thing that they can unilaterally dictate web standards but that's the reality so might as well take advantage of it for the better I guess.