There is a good amount of properties that should mostly be applied from a parent's point of view. We're talking stuff like grid-area in grid layouts, margin and flex in flex layouts. Even properties like position and and the top/right/left/bottom following it in some cases.

The main reason using classes isn't a great solution is that it completely breaks encapsulation in a confusing way, the paren't shouldn't be dictating anything, the component itself should. The parent can pass things and the child can choose to use them or not but that is different: control is still in the hands of the component itself, not an arbitrary parent.

The RFC is more appropriate because it does not allow a parent to abritrarily control anything below it, that responsibility still relies on the component itself. Just because people have been passing classes round and overriding child styles for years doesn't mean it is a good choice and isn't something we wnat to encourage.

the notion that any given component should be in charge of its own thing, and not do something outside of itself. I.e., loosely coupled components in a sandbox, not tightly coupled to something outside of its own scope.

Personally, I think class is too blunt an instrument — it breaks encapsulation, allowing component consumers to change styles that they probably shouldn't, while also denying them a predictable interface for targeting individual styles, or setting theme properties globally

...but ultimately the component itself has control over what is exposed, and can specify its own fallback values using normal CSS custom property syntax:

A component should be in complete control of itself. Not only should a component's styles not leak out but other component's style should never leak in. Consider this 'Encapsulation part 2' if you will. When writing a component, you have certain guarantees that not only will the styles you write be contained within the component, but nothing from the outside should affect you either. You have a certain confidence that if it works in isolation, then it will continue to work when embedded within a complex application.