Redirect to Preview mode if there is no network

dev/sw.js

@@ -97,7 +97,16 @@ self.addEventListener('fetch', function(event) {
         } else {
           queue
             .push(function () {
-              return fetch(event.request);
+              return fetch(event.request)
+                .then(function (response) {
+                  return response;
+                }, function (err) {
+                  // Redirect to Preview mode if there is no network
+                  if ( ! url.searchParams.get("preview")) {
+                    url.searchParams.append("preview", "True");
+                    return Response.redirect(url);
+                  }
+                });
             });
         }