[stephen_g]

There is already one - it’s called RTA (‘Restricted to Adults’) and is a meta tag that should filter out sites if you have parental controls enabled. I expect it’s one of the things Google etc. look at for SafeSearch to remove those results too (among other checks).

Of course, the irony is that only those that would label their sites as RTA already (so are already easily filtered) would comply with any ID requirement, so these kinds of laws achieve very little!