Fortunately most browsers already have some measures to prevent that (https://developer.mozilla.org/en-US/docs/Web/CSS/Privacy_and...), the demo avoids automating the process altogether, and relies on tricking the visitor into 'voluntarily' telling if they've visited a site.

I meant you can use CSS on the checkbox once it is checked, there is no need to actually submit a form. I understand that you can't use CSS directly on the :visited selector.

Ah I see, that makes sense.