Polyfills emulate service workers as they were not actually implemented in any browser. So they have to fall back on App Cache. But now they can seamlessly use Chrome's native service workers while still relying on polyfills for other browsers.
Emulating service worker with App Cache sounds... very hard. Do you have more info on this specifically? I've got an app that's using app cache now and I'd like to gracefully transition it to using service workers as they become available.
It's possible I misunderstood it then. I was at a talk by an FT engineer. Here's his course that he referenced in his talk: https://github.com/matthew-andrews/workshop-making-it-work-o....
You're right that maybe he was talking about what it is possible not what they did.
Polyfills emulate service workers as they were not actually implemented in any browser. So they have to fall back on App Cache. But now they can seamlessly use Chrome's native service workers while still relying on polyfills for other browsers.