That's a good idea. There could be two headers, the existing RTA header that adult sites use today [1] and another static header that explicitly states there shall be no adult content.

[1] - https://www.shodan.io/search?query=RTA-5042-1996-1400-1577-R... [THESE ARE ADULT SITES, NSFW]

Yes, the RTA header was primarily a solution specific to porn sites. The broader problem is that parental controls don't have reliable standardized signals to filter on which has led to the current nonfunctional mess.

So ideally you want a standardized header that can be used to self classify content into any number of arbitrary and potentially overlapping categories. The presence of that header should then be legally mandated with specific categories required to be marked as either present or absent.

So for example HN might be "user generated T, social media T, porn F" or similar with operators being free to include arbitrary additional categories (but we know from experience that most of them won't).

While this would be required by law, I imagine browser vendors might also drop support to load sites that don't send the header in order to coerce global compliance.

I always love seeing pros and cons of whitelist vs blacklist sorts of strategies in different scenarios.