Stale-While-Revalidate

A Cache-Control extension directive (defined in RFC 5861) that allows a cache to serve a stale response while asynchronously revalidating it in the background. The syntax is stale-while-revalidate=<seconds>, specifying how long past max-age the stale content can be served. This pattern provides instant responses to users while keeping content fresh. It is widely used by CDNs and supported by modern browsers.