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chromium/third_party/blink/web_tests/external/wpt/html/webappapis/dynamic-markup-insertion/document-write/module-tla-promise-iframe.html 

<!doctype html>
<script type=module>
await new Promise(resolve => {
  window.parent.document.test.step_timeout(resolve, 0);
  document.write("document.write body contents\n");
  document.close();
  window.parent.document.dispatchEvent(new CustomEvent("documentWriteDone"));
});
</script>

Initial body contents