<html>
<head>
  <script>
    function start() {
      const xhr = new XMLHttpRequest();
      const observer = new ResizeObserver(entries => {
        xhr.open('GET', '', false);
        xhr.send();
        typeof entries[0].borderBoxSize;
        typeof entries[0].contentRect;
        typeof entries[0].borderBoxSize;
      })
      observer.observe(document.getElementById('list'), {});
      window.close();
    }

    document.addEventListener('DOMContentLoaded', start);
  </script>
</head>
<body>
  <li class="" id="list">
</body>
</html>
