Some at-rules are only valid within a certain other at-rule, e.g. margin rules within @page, or @swash (etc) within @font-feature-values.

So, how do these evaluate?

@supports at-rule(@swash) {}
@supports at-rule(@top-left) {}

It seems unwise to just ignore context, since a given @foo may be specified to exist in more than one context, with different meanings (and possibly different descriptors being valid).

This was discussed during #2463, but I don't see a conclusion.