Variable fonts and stretchy operators

cc @drott @behdad @SergeyMalkin @bkardell @tiroj

Opening this for the record...

See https://github.com/stipub/stixfonts/issues/125#issuecomment-420394717 (especially the video)

The main advantages would be that

• It allows to reduce file size, which is probably interesting for math web fonts.
• We could reuse glyph stretching existing code (but of course we would still need to continue to support existing fonts anyway).

The issue was raised during the web engines hackfest 2017 ( https://github.com/Igalia/webengineshackfest/wiki/2017#mathml-fonts ) and IIRC Behdad mentioned that main concern is that math stretching starts from a target stretch size while variable fonts starts from a variation-axis value. In general, we cannot guess which axis value will provide the proper target size without more assumptions on how the font is designed. Additionally, math font designers don't necessarily want to perfectly match a target size but sometimes just to reach something a bit larger than the target size. This is typically the case for the smallest sizes.

So I think the MathVariant subtable should be extended to somewhat have a mapping between target stretch size ranges and the proper glyph + variation-axis + variation-axis value combination:
https://docs.microsoft.com/en-us/typography/opentype/spec/math#mathvariants-table

Once this is standardized in the Open Font Format, we can add something in
https://mathml-refresh.github.io/mathml-core/#size-variants-for-operators-mathvariants

Start by reviewing the linked STIX Fonts discussion, the OpenType MathVariants table, and the MathML Core section on size variants for operators. Done would require an agreed mapping between target stretch ranges, glyphs, and variation-axis values, followed by corresponding standard and MathML Core updates.