alt Hacker NewsThat's incorrect. The existing RTA header is for marking porn as porn.[1][2] (In case English isn't your first language, "adult website" is a euphemism.)
An actually useful standard would need to be much more granular, and be designed as a whitelist not a blacklist, as I and previous commenters said.
[1]: https://wiki.whatwg.org/wiki/MetaExtensions#:~:text=Restrict...
I disagree. Restricted To Adults means Restricted To Adults. The header can literally be used on any website that one is self describing as for adults. One could put it on a site that sells firearms for example. It could be applied to a siteThere are no restrictions. Team Dang could put it on news.ycombinator.com if they so desired. There is nothing and nobody that could stop them. I put it on my blogs even though there is no porn. The header can be used on anything that serves up HTTP and the meta tag can be put on any HTML page that may not be appropriate for children.
Here [1] is the only external link that is relevant and don't take their banner out of context. This was originally by the adult entertainment industry to self label and self regulate but is now used on any site that is not appropriate for minors. Entertainment could be inappropriate humor for example. That does require porn. I would link to some inappropriate humor but that would be inappropriate.
"The RTA Label is free to use, voluntary, and universally available to any website that wishes to clearly and effectively label itself as being inappropriate for viewing by minors."
[1] - https://www.rtalabel.org/page.php