[css-values-5] Disallow mix/random/tree-counting functions in some contexts

[cdoublev]

Based on comments in #10879, any value defined in CSS Values and Units should be valid in any context other than a property value, unless otherwise specified.

mix, random, tree couting functions, need an element, but are not explicitly disallowed in other contexts.

I would say they should be invalid in media queries and any descriptor value, and valid in the prelude of keyframe and @container rules. (It seems easier to find use cases in the prelude of keyframe rules.)

Perhaps defining the concept of element-dependent (substitution) value could help clarifying this: such value would be invalid in all contexts except those "associated" to an element, or that do not need to resolve its value (feature queries). var(), attr(), toggle(), <whole-value>, would be element-dependent subsitution values.

I do not know if any of these substitution functions should/can be valid in the HTML srcset attribute (to replace <length>), but invalid in the media attribute. I guess none is currently accepted, not even math functions.

[fantasai]

Some types of mix functions are reasonable to allow anywhere, they just represent the interpolation between two values and that's fine. We could also define something reasonable for random() outside of an element context. Agree with defining some kind of concept for element-dependent functions, though, and making such invalid (if they don't have a reasonable fallback behavior).

[cdoublev]

Allowing them anywhere would require making some arguments (<'animation-timeline'>, per-element) invalid in these other contexts.