I don't like the OSA and associated regulations as much as the next person -- I think we could have gotten a long way by saying you need to include a X-Age-Rating in http responses and calling it a day. The law itself is incoherently long and it's very difficult to know what duties you have.
However, I don't see what the legal basis of Wikimedia's challenge is. The OSA is primary legislation, so can't be challenged except under the HRA, which I don't really see working. The regulations are secondary regulation and are more open to challenge, but it's not clear what the basis of the challenge is. Are they saying the regulations are outside the scope of the statutory authority (doubtful)? You can't really challenge law or regulation in the UK on the basis of "I don't like it".
X-Age-Rating would only work if the server could be sure of the jurisdictions under which the recipient was bound.
To continue the thought experiment though: another implementation would be to list up to N tags that best describe the content being served. You could base these on various agreed tagging systems such as UN ISIC tagging (6010 Broadcasting Pop Music) or UDC, the successor to the Dewey Decimal System (657 Accountancy, 797 Water Sports etc.) The more popular sites could just grandfather in their own tag zoologies.
A cartoon song about wind surfing:
It’s then up to the recipient’s device to warn them of incoming illegal-in-your-state content.There actually was a proposal/standard for this back in the day: https://www.w3.org/PICS/
> X-Age-Rating would only work if the server could be sure of the jurisdictions under which the recipient was bound.
That's no different to the current legislation.
The twitter API used to have a "illegal in France or Germany" field, which was used for known Nazi content.