Feature request: variable fonts (-> fewer files, smaller byte size, customisable)

[pacien]

Is your feature request related to a problem? Please describe.

Currently, KaTeX relies on many font files for each variant (about
{AMS, Caligraphic, Fraktur, Main, Math, SansSerif, Script, Typewriter, Size1, .., Size4}
× {upright, italic} × {regular, bold}).

This amounts to about 350KB of font files for a each format.
This is quite a heavy to load,
as well as causing many requests,
causing multiple repaints and flickering on page load.
(This even if the math is pre-rendered).

Furthermore, customising the font weight or amount of slant for example
requires rebuilding the whole font set.

Describe the solution you'd like:

Using a variable font would resolve both issues
by having a single font file
defining the variants and being customisable through CSS.

At the time of writing, this is fairly well supported (96% browser coverage):
https://caniuse.com/variable-fonts

See also:
https://fonts.google.com/knowledge/introducing_type/introducing_variable_fonts

[edemaine]

I don't think combining the existing fonts into one font will make them smaller. But plausibly there are some variable fonts that support e.g. roman and bold math from a common source, which could halve the size. But I don't know whether any such variable fonts exist yet, with all of LaTeX's mathematical symbols... Do you know one?