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chromium/third_party/blink/web_tests/wpt_internal/prerender/resources/subapp.html 

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/speculation-rules/prerender/resources/utils.js"></script>
<script>

assert_true(document.prerendering);

const params = new URLSearchParams(location.search);
const uid = params.get('uid');

async function listSubApps() {
  const bc = new PrerenderChannel('prerender-channel', uid);
  try {
    const _ = await navigator.subApps.list();
    bc.postMessage('unexpected success');
  } catch (err){
    bc.postMessage(err.name);
  } finally {
    bc.close();
  }
}

listSubApps();

</script>