chromium/third_party/blink/web_tests/http/tests/text-autosizing/wide-iframe.html

<!DOCTYPE html>
<html>
<head>

<meta name="viewport" content="width=800">
<style>
body { width: 800px; margin: 0; }
</style>

<script src="resources/autosizingTest.js"></script>
<script>
  if (window.testRunner)
    testRunner.waitUntilDone();

  function done() {
    if (window.testRunner) {
      requestAnimationFrame(()=> {
        requestAnimationFrame(()=> {
          testRunner.notifyDone();
        });
      });
    }
  }
</script>

</head>
<body>

  <iframe
      onload="done()" style="width: 3200px; height: 50%; border: 0"
      src="http://localhost:8000/text-autosizing/resources/wide-iframe.html">

</body>
</html>