Intent to unship: mathvariant other than <mi mathvariant=normal>

[Frédéric Wang]

As of 2023-07-27, I intend to deprecate the MathML mathvariant
attribute, except when used on the `<mi>` element. Status in other
browsers is not supported in Chrome 109, supported in Safari.

Bug to deprecate: https://bugzilla.mozilla.org/show_bug.cgi?id=1845461

This is to align with the MathML Core specification:
https://w3c.github.io/mathml-core/#the-mathvariant-attribute where these
cases are written using the proper Unicode character (fraktur,
double-struck...) instead. For example <mi mathvariant="fraktur">x</mi>
can be replaced with <mi>#x1D51E;</mi>.

However, it is not clear yet how much this feature is used, so the plan
is to collect some telemetry data first and send some deprecation
warning to the console.

See also the related intent to ship:
https://groups.google.com/a/mozilla.org/d/msgid/dev-platform/753d2f38-ef67-8371-0f48-c4818961abf5%40igalia.com

--
Frédéric Wang