[yusyusyus]

Back in the day, during the short-lived reign of Communications Decency Act's provisions for dealing with pornographic material and minors, there emerged a site called sexkey.com that would provide a sort of SSO experience for doing age verification. That is, one would verify their age with sexkey.com and then, at participating sites, one would do a lil SSO bounce to verify age.

When the CDA's porn provisions were struck down, the sort of industry argument was that they'd use the PICS site ratings and the content could be blocked in proxy/client side. This made a lot of sense in the context of the V-chip mandates of the 90's. AFAIK browsers stopped supporting this a long time back.