In order to be compatible with legacy web content when interpreting something like Content-Type: text/html; charset=latin1, tools need to use a particular set of aliases for encoding labels as well as some overriding rules. For example, US-ASCII and iso-8859-1 on the web are actually aliases for windows-1252, and a UTF-8 or UTF-16 BOM takes precedence over any other encoding declaration. The WHATWG Encoding standard defines all such details so that implementations do not have to reverse-engineer each other.

This module implements the Encoding standard and has encoding labels and BOM detection, but the actual implementation for encoders and decoders is Python’s.