At least when following this example the site should work just fine when JS is disabled, which was the main issue I had when I tried following google's (?) tutorial on how to use service workers to make webapps work offline. That one revamped the whole site into a JS app powered by service workers, with a blank white page remaining without JS. Not the case here.

I don't think this approach here should cost much performance. It is just a thread checking a cache (that will be on disk) on page load.