Early Hints is a recent addition to the HTTP Informational response (1xx) status codes. Information response codes are temporary status codes used to inform the client about the status of the request, while the server is processing the request to send the final response code (2xx-5xx).

Early Hints is specifically used to pass information on the resources that may be preloaded by the client. The client will eventually need these resources when it renders the final response from the server.

stale-while-revalidate helps developers balance between immediacy —loading cached content right away— and freshness —ensuring updates to the cached content are used in the future.

How To Remove Unwanted HTTP Response Headers

Un doute sur un code HTTP ? https://http.cat/[status_code] et voilà !

Excellente synthèse de ce qu'est et ce que doit être réellement une API RESTful.