I just witnessed the cached offline page which comes with this extension work in the wild for the first time. My Internet connection temporarily stopped working and sure enough, when I refreshed the page at my forum, instead of getting a default browser connection error page, I got the PWA offline cached page. Which is kind of nice.
Now, the offline page that comes with this extension is extremely minimal. If you're pretty good with HTML/CSS/JavaScript you can edit the offline.html file that comes with this extension and make something extravagant if you wanted to. Maybe a fun little game for users to play while their Internet, or the site, is down. 🙂