HTML: `charset`

# HTML: `charset`

Use an ASCII case-insensitive match for the character encoding's label;
for example: `UTF-8`.

# References

[4.2.5. The meta element](https://www.w3.org/TR/2021/SPSD-html52-20210128/document-metadata.html#the-meta-element):

> The charset attribute specifies the character encoding used by the
> document. This is a character encoding declaration. If the attribute
> is present in an XML document, its value must be an ASCII
> case-insensitive match for the string "utf-8" (and the document is
> therefore forced to use UTF-8 as its encoding).

[4.2.5.5. Specifying the document’s character encoding](https://www.w3.org/TR/2021/SPSD-html52-20210128/document-metadata.html#specifying-the-documents-character-encoding):

> The character encoding name given must be an ASCII case-insensitive
> match for one of the labels of the character encoding used to
> serialize the file. [ENCODING]

[ENCODING](https://www.w3.org/TR/2021/SPSD-html52-20210128/references.html#biblio-encoding):

> Anne van Kesteren, Joshua Bell, Addison Phillips. Encoding. CR. URL:
> <https://www.w3.org/TR/encoding/>

<https://www.w3.org/TR/encoding/> redirects to
[Encoding](https://encoding.spec.whatwg.org/).

For the `UTF-8` encoding,
[4.2. Names and labels](https://encoding.spec.whatwg.org/commit-snapshots/75b988c17cc2b90266e69526f399c7916c3e0ef0/#names-and-labels)
lists the following labels:

> * "unicode-1-1-utf-8"
> * "unicode11utf8"
> * "unicode20utf8"
> * "utf-8"
> * "utf8"
> * "x-unicode20utf8"

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