Revalidating is optional. A browser MAY not revalidate if it likes so, unless you use `must-revalidate`: https://tools.ietf.org/html/rfc2616#section-14.9.3 htt

Revalidating is optional. A browser MAY not revalidate if it likes so, unless you use `must-revalidate`: https://tools.ietf.org/html/rfc2616#section-14.9.3 https://tools.ietf.org/html/rfc2616#section-14.9.3 > A cache MAY be configured to return stale responses without validation, but only if this does not conflict with any "MUST"-level requirements concerning cache validation (e.g., a "must-revalidate" cache-control directive). https://tools.ietf.org/html/rfc2616#section-14.9.4 https://tools.ietf.org/html/rfc2616#section-14.9.4 > must-revalidate > Because a cache MAY be configured to ignore a server's specified expiration time, and because a client request MAY include a max-stale directive (which has a similar effect), the protocol also includes a mechanism for the origin server to require revalidation of a cache entry on any subsequent use.