#2463 resolved to add at-rule(@foo; descriptor: value), but I think there are several cases where it's not clear if something should be considered a descriptor or not. For example:

• Do @page (/margin) rules hold descriptors or properties? (They do not apply to elements, in which case they are arguably descriptors.)
• The accepted things within @position-try are described as both properties and descriptors by css-anchor-position-1.
• I'm sure there are other cases.

We should probably just avoid this problem by instead specifying at-rule(@foo; <declaration>), and evaluate to true whenever that declaration (whatever it is) would be valid within @foo.