declarative-shadow-dom-write-to-iframe.html

<!DOCTYPE html>
<meta charset=utf-8>
<title>`document.write` on inner iframe handles declarative shadow DOM</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<iframe></iframe>
<script>
promise_test(async () => {
  await new Promise(res => window.addEventListener("load", res));
  let elem = document.querySelector("iframe");
  elem.contentDocument.write(`
    <div>
      <template shadowrootmode="open"><slot></slot></template>
      <p>Test</p>
    </div>
  `);
  let container = elem.contentDocument.querySelector("div");
  assert_true(!!container, "write should occur");
  assert_true(!!container.shadowRoot, "write should create shadowroot");
  assert_equals(container.innerText, "Test", "div should still contain text");
}, "`document.write` on inner iframe handles declarative shadow DOM");
</script>

chromium/third_party/blink/web_tests/external/wpt/shadow-dom/declarative/declarative-shadow-dom-write-to-iframe.html