Another funny thing here is that they say “but not limited to” (the listed encodings), but then say “must not support other encodings” (than the listed ones).

> the encodings defined in Encoding, including, but not limited to

where "Encoding" refers to https://encoding.spec.whatwg.org (probably that should be a link.) So it just means "the other spec defines at least these, but maybe others too." (e.g. EUC-JP is included in Encoding but not listed in HTML.)