The OpenType spec includes support for variable weights, added in 2016.

This is yet another feature it borrowed from TrueType GX, which in turn was a part of Mac's QuickDraw GX dating back to mid 90s.

[1] https://en.wikipedia.org/wiki/Variable_fonts

[2] https://en.wikipedia.org/wiki/QuickDraw_GX#TrueType_GX

[3] https://webkit.org/blog/7051/variable-fonts-on-the-web/