That works fine for your xhr data (and is what I do now), but is idiosyncratic to your library (note that detecting online state, responding to online state changes, etc all add complexity to this seemingly simple module, as does managing cache expiry).

You can't do anything about your top level index.html file (or the main js and css assets) though. If the user starts offline the second time they use your app, there's no guarantees about those bootup assets being loaded into cache, you'll just get the "you're offline" screen instead.

Since we are talking about hooking the lifecycle of a page, there's no good way to polyfill this with a library. That initial request needs to be serviced, offline, before you can load your polyfill library.