Well, if you're going to go this way, you'd use PICS tags of some kind probably. But the problem is that if every webmaster has to classify their own content, there have to be very clear-cut rules so they don't get it wrong. The problem is that if you make the rules that simple, they're not much use, and if you make them more complex (a la ICRA/RSACi) they become too complicated for most people to apply. (Certainly too complicated if one faces legal penalties for getting it wrong.)
Basically, self-labelling of content is a non-starter. Adult sites will self-label, kids sites may self-label, but that leaves the huge bulk of other material, much of it (in some people's eyes) unsuitable for children, much of it aimed at children.
So self-labelling is only useful *positively*, as an aid to helping people find content they want, whether sites suitable for children (whitelist) or porn sites.