Dwarves Memo

The Cache-Control HTTP header holds directives (instructions) for caching (https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching) in both requests and responses.
stale-while-revalidation is an extension Cache-Control directives which are not part of the core HTTP caching standards document.
stale-while-revalidation indicates the client will accept a stale response, while asynchronously checking in the background for a fresh one. The seconds value indicates how long the client will accept a stale response. Note that the time does not start at the time of the request itself, but, for example, after max-age has elapsed.
Support for setting stale-while-revalidate alongside max-age in your Cache-Control response header is available in Chrome 75 (https://chromestatus.com/feature/5050913014153216) and Firefox 68(https://bugzilla.mozilla.org/show_bug.cgi?id=1536511).

Cache-Control: stale-while-revalidate=<seconds>

When you set the HTTP response header like below, this setting means

Cache-Control: max-age=1, stale-while-revalidate=59

If a request for the time is repeated within the next 1 second, the previously cached value will still be fresh, and used as-is, without any revalidation.
If a request is repeated between 1 and 60 seconds later, then the cached value will be stale, but will be used to fulfill the API request. At the same time, "in the background," a revalidation request will be made to populate the cache with a fresh value for future use.
If a request is repeated after more than 60 seconds, then the stale response isn't used at all, and both fulfilling the browser's request and the cache revalidation will depend on getting a response back from the network.