index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="Pragma" content="no-cache" /> <!-- http 1.0 -->
    <meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate" /> <!-- http 1.1 -->
    <meta http-equiv="Expires" content="0" /> <!-- proxy -->
    <title>Flipper!</title>
    <script type="text/javascript">
      window.onload = function() {
        let c = `; ${document.cookie}`;
        let v = c.split("; flipper=");
        if (v=="; ") {
          c = "Hello";
        } else {
          c = v[1];
        }
        let l = document.querySelector("body");
        l.textContent = c;
        if(c=="Hello") {
          c = "World";
        } else {
          c = "Hello";
        }
        document.cookie = `flipper=${c}`;
      };
    </script>
  </head>
  <body>
  </body>
</html>