service worker ready&quot;);
      await sleepMs(100);
      let response = await fetch(&quot;serviceworker-read&quot;);
      return await response.text();
    }

result, same first party: &lt;html&gt;
&lt;head&gt;&lt;title&gt;404 Not Found&lt;/title&gt;&lt;/head&gt;
&lt;body&gt;
&lt;center&gt;&lt;h1&gt;404 Not Found&lt;/h1&gt;&lt;/center&gt;
&lt;hr&gt;&lt;center&gt;nginx/1.18.0 (Ubuntu)&lt;/center&gt;
&lt;/body&gt;
&lt;/html&gt;
&lt;!-- a padding to disable MSIE and Chrome friendly error page --&gt