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The source .rst for 
<https://docs.python.org/3/reference/lexical_analysis.html#encoding-declarations>
has at the end:
.. XXX there should be a list of supported encodings.
While I believe this is impractical, there could be a link to
https://docs.python.org/3/library/codecs.html#standard-encodings
-- and possible subsubsections that follow. Are the latter supported for Python code?

I have added link to Standard Encodings : https://docs.python.org/3/library/codecs.html#standard-encodings
and Python Specific Encodings : https://docs.python.org/3/library/codecs.html#python-specific-encodings 

You can remove the ".. XXX" line; I understand it is just like a TODO comment, and with this fixed it would no longer be relevant. I suggest putting the links next to the sentence that ends ". . . the encoding name must be recognized by Python."
The links should be internal links, rather than hard-coded Internet links to another version of the documentation. See <https://docs.python.org/devguide/documenting.html#cross-linking-markup>. The Standard Encodings section already has a label already set up for you to use :)

PEP 263 doesn’t say exactly what encodings are supported. It mentions Shift JIS is supported, but UTF-16 is not. Only UTF-8 is allowed if the file starts with a UTF-8 BOM. I guess many of the Python-specific text encodings from the second section may be supported. Probably any text encoding in general, as long as the first one or two lines can "rougly" be parsed in ASCII.

I am writing a PR with an internal link. I decided that linking to Standard Encodings is sufficient. The Python Text Encodings section follows that. Anyone wanting to use any of those can experiment to see which work.

New changeset 01be5d6446abbdd95d0c18bd19a58a62b05568d8 by Terry Jan Reedy in branch 'main':
bpo-24563: Link encoding names to encoding declarations (GH-32274)
https://github.com/python/cpython/commit/01be5d6446abbdd95d0c18bd19a58a62b05568d8

New changeset f4e280d8c5461fa5e928fa07f6cd1779a4ac43ba by Miss Islington (bot) in branch '3.9':
bpo-24563: Link encoding names to encoding declarations (GH-32274)
https://github.com/python/cpython/commit/f4e280d8c5461fa5e928fa07f6cd1779a4ac43ba

New changeset ce6af314ca8529d5ed0f307deb9c33029a69f4cb by Miss Islington (bot) in branch '3.10':
bpo-24563: Link encoding names to encoding declarations (GH-32274)
https://github.com/python/cpython/commit/ce6af314ca8529d5ed0f307deb9c33029a69f4cb