It's a bummer that this requires the :not pseudo-selector. I suppose it's not particularly an issue here, but selectors that might potentially have several modifiers will need extra care; scoping a selector for a component's "default" state could require multiple :not checks that could get difficult to manage. Hopefully that's a relatively rare case though.

(And I suppose it's more likely to be the case in tests like this; it's probably possible to architect without it being a concern in the CSS itself, and especially if some of the Emotion explorations move forward.)

Yeah, I see this as mostly a use-case we'd want to isolate within tests, where we're explicitly wanting to distinguish between the default and the variant.

In real-world application, we should typically want to treat the variant as something of an additive layer upon the default, even to the extent of unsetting/overriding properties from those defaults, vs. ever feeling the need to use :not( .is-variant ) in CSS. If we found ourselves needing to use :not, it probably raises a bigger question of whether we can really consider there to be a default (vs. other named variants, or treating them as wholly different elements).