What is a Warning header?
A now-deprecated way to carry additional information about the status of a message, particularly around caching.
Warning used to flag caveats about a response - most often that it was served stale from a cache - but it's been formally removed from the current HTTP specification and modern browsers largely ignore it. You'll still occasionally spot it on older infrastructure or CDNs that haven't updated their caching layer, which can be a useful signal if you're auditing a website's headers for anything overdue for modernisation.
Common use case
Historically used by caches to flag a stale response with codes like 110 (Response is Stale), giving clients a machine-readable reason without changing the actual status code.
Example
Warning: 110 - "Response is Stale"History
Introduced in HTTP/1.1 (RFC 2068, 1997) and refined in RFC 7234 (2014).
Deprecated / legacy status
Removed outright in RFC 9111 (2022), the current HTTP caching specification, because in practice almost no client ever acted on its value - it was informational clutter that never earned its keep.