I agree that most design teams are not going to actually write CSS. However, it seems reasonable to me to expect them to organize and communicate their design in terms of classes.

I see two advantages to that approach:

- It requires the design team to take responsibility for structuring the design. E.g. determining parts of the UI that share the same design and defining a hierarchy of design elements.

- It makes the work that the design team is creating for the engineers more transparent. Communication can be something like "this new wireframe requires 5 new classes" rather than something vague like "just tweak the styles we used for the other components to fit this new wireframe".