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chromium/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/fetch-request-xhr-sync-iframe.html 

<!DOCTYPE html>
<title>Service Worker: Synchronous XHR is intercepted iframe</title>
<script>
'use strict';

function performSyncXHR(url) {
  var syncXhr = new XMLHttpRequest();
  syncXhr.open('GET', url, false);
  syncXhr.send();

  return syncXhr;
}
</script>