I think ”persist the input in the URL query string” could also be doable with static assets alone. The last <script> block in foo.html would need to be replaced with something like:

  <script>
    // calculate output and update query string when any input changes
    a.oninput = b.oninput = c.oninput = function () {
      z.value = foo(
        Number(a.value),
        Number(b.value),
        Number(c.value),
      );
  
      var params = new URLSearchParams();
      if (a.value) params.set('a', a.value);
      if (b.value) params.set('b', b.value);
      if (c.value) params.set('c', c.value);
      history.replaceState(null, '', '?' + params);
    };
  
    // fill in input from query string when page is loaded
    var params = new URLSearchParams(location.search);
    a.value = params.get('a');
    b.value = params.get('b');
    c.value = params.get('c');
    a.oninput();
  </script>