[mediaqueries-5] @import motivating example doesn't work?

[cvrebert]

Spec: https://drafts.csswg.org/mediaqueries-5/#custom-mq

The intro paragraphs give as motivation for custom media queries:

the same media query may be used in multiple places, such as to qualify multiple @import statements.
[...] In this way, a media query used in multiple places can instead be assigned to a custom media query,

This makes it sound like we could write something like:

@custom-media --narrow-window (max-width: 30em);
@import url("mobile.css") (--narrow-window);
@import url("more-mobile.css") (--narrow-window);

However, that doesn't actually seem legal, per css-cascade (emphasis mine):

Any @import rules must precede all other at-rules and style rules in a style sheet (besides @charset, which must be the first thing in the style sheet if it exists), or else the @import rule is invalid.

[tabatkins]

Hm, good point. We'll need to weaken that statement to allow for relevant MQ-defining at-rules (and others with similar effects) before an @import.

[cvrebert]

So assuming that were made legal, what would then prohibit:

/* foo.css */
@custom-media --my-mq true;
@import url("bar.css") (--my-mq);

/* bar.css */
@custom-media --my-mq false;

?