title: How are "The app is currently unreachable" error pages served?
body: <p>I've noticed while using Google Chrome that if a website such as Facebook, YouTube or Google Docs is inaccessible a certain error page that says "<strong>The app is currently unreachable.</strong>" is served, like the one below.</p>
  
  <p><img src="https://i.stack.imgur.com/qaYQP.png" alt="enter image description here"></p>
  
  <p>Is this internal to Chrome or is it part of an offline web app manifest file?</p>