I spent about half an hour trying to figure out why some JSON in my browser was rendering è incorrectly, despite the output code and downloaded files being seemingly perfect. I came to the conclusion that the browsers (Safari and Chrome) don't use UTF-8 as the default renderer for everything and moved on.
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.)