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chromium/third_party/blink/web_tests/external/wpt/fenced-frame/resources/cookie-access.https.html 

<!DOCTYPE html>
<script src="utils.js"></script>
<title>Page loaded in a frame in a fenced frame tree</title>
<script>
  // This page is loaded either in an iframe or a fenced frame
  // nested inside a root fenced frame.
  document.cookie = 'G=nested_in_fenced_frame; SameSite=Lax';
  const [cookie_value_key] = parseKeylist()
  const cookie_value = document.cookie;
  writeValueToServer(cookie_value_key, cookie_value);
</script>