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chromium/third_party/blink/web_tests/http/tests/inspector-protocol/resources/frame-testing-dom-storage.html 

<!DOCTYPE html>
<script>
  const itemPreSet = window.localStorage.getItem('testKey');
  window.parent.postMessage("item value: " + itemPreSet, '*');
  window.localStorage.setItem('testKey', 'testItem');
</script>