226 IM Used

Sent only the difference — the delta response the web never adopted.

RFC 3229, from 2002, proposed delta encoding: the client says it accepts diffs with A-IM: vcdiff and the server returns 226 with only what changed since the version the client already holds, identified by its ETag.

The idea lost to plain caches and compression, dumber and easier. 226 remained a museum piece in the strict sense: registered at IANA, described in an RFC, emitted by almost nothing in production.

HTTP/1.1 226 IM Used
IM: vcdiff
ETag: "489uhw"
Cache-Control: no-store

Not the file: only what changed since the last visit.