How do you define "porn" or whatever material requires the xxx designation? I don't know how to define it, but I know it when I see it. :-)
Seriously though, it would be no different than how it is defined for movies, magazines, etc. Obviously that has to be some definition for those.
The real trick is in the enforcement. Governments can come up with definitions of pornography that satisfy most of the population in most cases (no definition is ever going to completely clear and consistent.) So we can distinguish between the strip club and Hooters for example. The basic questions are:
- Who decides?
- How is the Web monitored for compliance?
If this is a purely voluntary, then it's worthless. It would be like asking strip clubs to voluntarily not open near schools. Maybe they will and maybe they won't.