Get list of resources needed for page for PWA precaching

[FuJa0815]

I'm building a PWA and want to present the user a "You are offline"-Page whenever they are offline. I've made a /offline route with text and a bit of styling and an image.

Now I tried registering it in the service worker to precache the offline page. I've registered the /offline page as a fallback to use when the user is offline, but things like the stylesheet or the image are not precached. How do I get a list of resources used by a page? I get all files from the $service-worker resource, but I don't want to precache everything, just the resources used by the /offline page.

Thanks!