chromium/third_party/blink/web_tests/fast/parser/force-plaintext.html

<!DOCTYPE html>
<p>plaintest test with postMessage</p>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script>
var t = async_test("Do not parse plain text as HTML");
window.addEventListener('message', function() {
  assert_unreached("Child should not be executable");
});
var iframe = document.createElement('iframe');
iframe.src = "data:text/plain,<script>parent.postMessage('Hello','" + location.origin+"');<\/script>";
iframe.onload = () => t.done();
document.body.appendChild(iframe);
</script>