[css-fonts-4] Clarify @font-palette-values at-rules as unavailable in shadow trees

[drott]

Similar to #5167 I suggest we clarify what happens with @font-palette-value at-rules in shadow-trees, i.e. similar to that issue clarifying

"A @font-palette-values rule is invalid if it occurs in a stylesheet inside of a shadow tree, and must be ignored."

[drott]

Similarly, it may make sense to clarify if @font-palette-value rules are allowed to appear in @layer rules.

[drott]

Ping @litherum @svgeesus @jfkthame - do you have opinions on this? Can we clarify this question?

[jfkthame]

I guess this is fine for the time being. Will this include a "will likely change in the future" issue, though, similar to #5167? ISTM that in principle using @font-palette-value rules in shadow-trees should be perfectly reasonable, even if we're not able to allow it for now. (Or is there a reason to permanently exclude it?)

[tabatkins]

My answer from 2020 stands - we only disallow insofar as impls are not yet following the rules in Scoping. Once an implementation supports those, then they must support these at-rules in shadow trees, following these rules.

[litherum]

I don't know why font-palette-values should deserve any special treatment - it should follow whatever the behavior is for other at rules.