webencodings is a Python implementation of the WHATWG Encoding standard.

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 an UTF-8 or UTF-16 BOM takes
precedence over any other encoding declaration. The Encoding standard
defines all such details so that implementations do not have to
reverse-engineer each other.

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