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No, each square the user clicks could be a checkbox that is submitted to the backend as a form when they click done.

• kevincox 5y ago
  It could even just use CSS selectors to reveal an image or change a background image that results in a request to the backend.
  • varun_ch 5y ago
    Fortunately most browsers already have some measures to prevent that (https://developer.mozilla.org/en-US/docs/Web/CSS/Privacy_and_the_:visited_selector 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.
    • kevincox 5y ago
      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.
      • varun_ch 5y ago
        Ah I see, that makes sense.