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chromium/third_party/blink/web_tests/external/wpt/infrastructure/reftest/reftest_window_load.html 

<!DOCTYPE html>
<title>rel=match waits for window load</title>
<link rel=match href=green.html>
<script>
  window.onload = () => {
    const s = document.createElement('style');
    s.innerText = ':root {background-color:green}';
    document.body.appendChild(s);
  };
</script>