public/404.html

<!DOCTYPE html>
<html>
  <head>
      <meta charset="utf-8" />
      <title>404</title>
      <script>
        const segment = 1;
        //Gets the relative path and the hash of the URL.
        //sessionStorage.redirect = '/' + location.pathname.slice(1).split('/').slice(segment).join('/');
        sessionStorage.redirect = location.pathname.split('/').slice(0, 1 + segment).join('/');
        sessionStorage.hash = location.hash;

        //Forces the navigation back to the main page, since it's the only entry point that works.
        //location.replace(location.pathname.split('/').slice(0, 1 + segment).join('/'));
        location.replace('/' + location.pathname.slice(1).split('/').slice(segment).join('/'));
      </script>
  </head>

  <body>
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  </body>
</html>