src/third_party/web_platform_tests/html/dom/dynamic-markup-insertion/document-write/iframe_001.html

<!doctype html>
<title>document.write into iframe</title>
<script src="/resources/testharness.js"></script><script src="/resources/testharnessreport.js"></script>
<iframe id="test"></iframe>
<script>
test(
function() {
  var iframe = document.getElementById("test");
  iframe.contentDocument.write("Filler Text");
  iframe.contentDocument.close();
  assert_equals(iframe.contentDocument.body.textContent, "Filler Text");
});
</script>
<div id="log"></div>