[css-fonts-4] Accept all math functions in `@font-palette-values`

  > Functions such as `calc()`, `var()`, and `env()` are valid within the braces of a `@font-palette-values rule`.

https://drafts.csswg.org/css-fonts-4/#at-ruledef-font-palette-values

Obviously, I assume this means they are valid *descriptor values*, not descriptor names. But what is the reason (if any) to only accept `calc()` rather than all math functions?