title: Progressive Web App "does not work offline" error
body: <p>I have written a progressive web app, following all available guides and examples, but for some reason when I click the <code>Add to homescreen</code> button, I keep getting this mysterious error:</p>
  
  <pre><code>Site cannot be installed: does not work offline
  </code></pre>
  
  <p>The major difference between my PWA and the examples, is that mine is running purely in a non-root path of the domain, so I have had to add extra paths to the configs in various places so the app is restricted to the non-root folder.</p>
  
  <p>The <a href="https://developers.google.com/web/tools/lighthouse/" rel="noreferrer">Google Lighthouse</a> site doesn't help much either, giving a very similar message.</p>
  
  <p>Can anyone suggest what this error might be caused by?</p>