Content-Encoding representation header lists any encodings that have been applied to the representation (message payload), and in what order.
This lets the recipient know how to decode the representation in order to obtain the original payload format.
Content encoding is mainly used to compress the message data without losing information about the origin media type.
Note that the original media/content type is specified in the
Content-Type header, and that the
Content-Encoding applies to the representation, or "coded form", of the data. If the original media is encoded in some way (e.g. a zip file) then this information would not be included in the
Servers are encouraged to compress data as much as possible, and should use content encoding where appropriate. Compressing a compressed media type such as a zip or jpeg may not be appropriate, as this can make the payload larger.
// Multiple, in the order in which they were applied
Content-Encoding: deflate, gzip
A format using the Lempel-Ziv coding (LZ77), with a 32-bit CRC. This is the original format of the UNIX gzip program. The HTTP/1.1 standard also recommends that the servers supporting this content-encoding should recognize
x-gzipas an alias, for compatibility purposes.
A format using the Lempel-Ziv-Welch (LZW) algorithm. The value name was taken from the UNIX compress program, which implemented this algorithm. Like the compress program, which has disappeared from most UNIX distributions, this content-encoding is not used by many browsers today, partly because of a patent issue (it expired in 2003).
A format using the Brotli algorithm.
On the client side, you can advertise a list of compression schemes that will be sent
along in an HTTP request. The
Accept-Encoding header is used for
negotiating content encoding.
Accept-Encoding: gzip, deflate
The server responds with the scheme used, indicated by the
Content-Encoding response header.
Note that the server is not obligated to use any compression method. Compression highly depends on server settings and used server modules.
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