The general recommendation is to compress data whenever possible, but some types of resources, such as JPEG images, are already compressed, and using additional compression will have no effect.
If multiple encodings are applied to an entity, the content encodings MUST be listed in the order in which they were applied Additional information about the encoding parameters MAY be provided by other fields of the message header.
If the message doesn't contain the "no-transform" cache-control directive, and if the recipient can accept new encodings, then a non-transparent proxy may modify the content-coding of the entity.
- identity: no transformation is used, the default value for content coding.
- gzip: GNU zip format uses the deflate algorithm for compression.
- compress: UNIX "compress" program method, deprecated and replaced by gzip or deflate.
- deflate: compression based on the deflate algorithm.
- br: a modern, effective compression algorithm specifically designed for HTTP.
And the server response with Content-Encoding header.