[css-values-5] Express which keywords are valid in calculations in syntax definitions

[romainmenke]

Some examples:

color: rgb(from purple r g calc( b / 2 ));
height: calc-size(auto, calc(size / 2));

Currently there is no way to express in syntax definitions which keywords are valid in which context.
This can only be done in prose.

This would be useful for static analysis tools (i.e. linters)

With the addition of the boolean expression multiplier there is bit of a precedent to define and communicate such things.

Could we use this or something similar for math expressions?

<modern-rgb-syntax> = rgb( [ from <color> ]?
        [ <number> | <percentage> | <math-function[ r | g | b ]> | none]{3}
        [ / [<alpha-value> | <math-function[ alpha ]> | none] ]?  )

Where <math-function[ foo ]> is equivalant to calc( <calc-sum[ foo ]> ) | min( <calc-sum[ foo ]> ) | ...

<calc-size()> = calc-size( <calc-size-basis>, <calc-sum[ size ]> )