[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.

[lilles]

I was thinking they should be allowed in container queries, evaluating against the container element, but then the size query container may fall back to the viewport which is not an element.

[lilles]

@tabatkins @emilio @nt1m Any opinions on whether tree-counting functions like sibling-index() should be valid parse time in:

• @page properties
• descriptors for @font-face and other at-rules which do not apply directly to elements
• @container queries (this is particularly interesting since the container element can be a natural choice most of the time)
• @media queries

And if such functions are valid, what should they evaluate to.

[emilio]

I think it makes sense to disallow them to begin with...

Only container queries really have something resembling a sensible behavior IMO, and what should the fallback behavior be is not particularly clear...

Disallowing it gives us more leeway to decide on what the right behavior should be once there are concrete use cases for it...

[tabatkins]

Yeah, I'm fine with starting with them all disallowed, parse-time invalid. We can turn them on as requested.