[css-fonts-4] How to handle math functions in font feature value declarations?

[cdoublev]

The value of a font feature value declaration is internally stored and exposed as unsigned integer(s) (spec).

But math functions are accepted in any context, unless otherwise specified, and are resolved at computed value time or later. So what is the expected output below?

const sheet = new CSSStyleSheet
sheet.insertRule(`
  @font-feature-values name {
    @annotation {
      boxed: calc(1em / 1em);
    }
  }
`)
sheet.cssRules[0].annotation.get('boxed'); // ???

I see 3 options:

1. []: disallow math functions in font feature value declarations (by defining them with <number-token> whose type should be integer, instead of <integer>)
2. [1]: resolve math functions at parse time in font feature value declarations (assuming relative length can be resolved, #10853)
3. calc(1em / 1em): deal with CSSOMStrings instead of unsigned longs

I have a preference for 3. Even if math functions might not be useful in this context, 1 is restrictive and 2 is static. Besides, with @view-transition/types, they are the only descriptors whose attribute getter do not return a CSSOMString.