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chromium/third_party/blink/web_tests/external/wpt/html/browsers/history/joint-session-history/joint-session-history-child1.html 

<body>
  <a id="link" href="joint-session-history-child2.html">Child1</a>.
  <iframe id="grandchild"></iframe>
</body>
<script>
  window.onload = function() {
    var link = document.getElementById("link");
    var grandchild = document.getElementById("grandchild");
    var timer = window.setInterval(poll, 100);
    function poll() {
      if (grandchild.getAttribute("data-grandchild-loaded")) {
        window.clearInterval(timer);
        link.click();
      }
    }
    grandchild.src="joint-session-history-grandchild1.html";
  };
</script>