203 Non-Authoritative Information
A 200 with a warning on the frame: someone touched the response on the way.
It is success with a caveat: the request worked, but a transforming proxy along the way — one that recompresses images, injects notices, rewrites pages — modified the content, and the 203 declares that what arrived is no longer the original.
It was the code of mobile carriers squeezing images over 3G to save bandwidth. With end-to-end HTTPS, the middlebox lost access to the content — and 203 lost its job. It remains registered, waiting for a world with less encryption.
HTTP/1.1 203 Non-Authoritative Information
Via: 1.1 proxy-da-operadora
Content-Type: image/jpeg
Content-Length: 24817