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chromium/chrome/test/data/webapps_integration/wco/basic.html 

<!DOCTYPE html>
<html>
<!--Test page with window controls overlay support-->
<head>
  <title>Site WCO</title>
  <link rel="icon" href="basic-48.png">
  <script src="/webapps_integration/test_utils.js"></script>
  <script>
    addManifestLinkTag("manifest_window_controls_overlay.json");
  </script>
</head>
<body onload="startWorker('/webapps_integration/wco/service_worker.js', { scope: '/webapps_integration/wco/' })">
  <h1>Site WCO</h1>
</body>
</html>