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chromium/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/window-opener.html 

<!DOCTYPE html>
<meta charset="utf-8"/>
<meta name="referrer" content="origin">
<script>
function onLoad() {
  self.onmessage = evt => {
    if (self.opener)
      self.opener.postMessage(evt.data, '*');
    else
      self.top.postMessage(evt.data, '*');
  }
  const params = new URLSearchParams(self.location.search);
  const w = window.open(params.get('target'));
  self.addEventListener('unload', evt => w.close());
}
self.addEventListener('load', onLoad);
</script>