Service Workers
Service Workers are Web Workers that can proxy network requests. I recommend web.dev's article for a not-too-short, not-too-long intro.
Save for offline
This website uses service workers to implement offline features. When you click "Save for offline" or (press the S key) on this or any other page, the response from the server will be stored using the Cache API.
If you load the page offline, it should still be there!
Each request on this site is intercepted by a service worker, using a "network-first" strategy. If the resource is found on the server, it'll send the latest. Otherwise, it'll look in the cache.
More to come here...