[css-env-1] should define what happens when a predefined variable specific to certain media doesn't match the medium

[dbaron]

In #2629#2807 we're starting to define predefined variables, and these have an associated medium.

Something ought to say what this means. In particular, I suspect what should happen is that if CSS is being used in a medium that doesn't match the predefined variable, then the predefined variable shouldn't be present. For example, in speech media, env(safe-area-inset-top) should behave the same as env(variable-that-does-not-exist).

This probably ought to be defined where the concept of predefined variables is defined.