[css-fonts] How should relative-length values be interpreted when used in @font-feature-values descriptors?

[weinig]

How should relative-length values be interpreted when used in @font-feature-values descriptors?

The grammars for @font-feature-values descriptors allow the use of relative-length values due to the fact that calc() allows for them.

The grammar for the declaration values is given as:

Each declaration’s value must match the grammar <integer [0,∞]>+, or else the declaration is invalid and must be ignored

For example, that would allow:

@font-feature-values foo {
    @swash { pretty: 1; cool: calc(10 - (5 * (10em - 2px)); }
}

So the question is, what does 10em evaluate to? @font-palette-values contains this text:

Math functions, such as calc(), and also var(), and env(), are valid within descriptor values in a @font-palette-values rule. They are evaluated within the context of the root element. Relative units are also evaluated within the context of the root element.

but I can't find any similar text for @font-feature-values.

[fantasai]

I can't think of a single use case for this, so if it's easier on implementations I think we can disallow it.

[tabatkins]

I have the same response here as in #10877 (comment)