Codebase Browser
chromium
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chromium/third_party/blink/web_tests/external/wpt/cookies/samesite/resources/iframe.document.html 

<!DOCTYPE html>
<script>
  var value = "" + Math.random();
  document.cookie = `dc_samesite_strict=${value}; secure; sameSite=strict; path=/`;
  document.cookie = `dc_samesite_lax=${value}; secure; sameSite=lax; path=/`;
  document.cookie = `dc_samesite_none=${value}; secure; sameSite=none; path=/`;
  parent.postMessage({value}, "*");
</script>