I cannot see anything of this nature working. While some porn providers may operate within the law (of the jurisdiction in which they are located), it is a fairly shady industry and many website operators are likely to have no interest in marking all of their media, especially if they happen to be located outside the UK.
This idea also fails to address the problem of social media/media sharing/messaging sites/apps being used to transmit porn, not least by teenagers themselves.
I reckon the only effective method is to constrain use on the user devices. The snag is, many of the parental control apps/features are not that difficult to defeat. It doesn't help that many ISPs still insist that customers use the ISP-supplied broadband routers, which typically have limited parental control features.
Biting the hand that feeds IT
