Don't even bother having the website ask the browser anything at all. Just have the website TELL the browser the content is intended for adults via HTTP header and let the browser decide to display it or not depending on parental controls.

Yeah, that's even better.

Multiple proposal for this exist for decades[1][2]

The RTA one was even pushed by the porn industry and is already in place in majors websites[3]

[1]:https://www.rtalabel.org/

[2]:https://icra.org/webmasters/

[3]:https://en.wikipedia.org/wiki/Association_of_Sites_Advocatin...