> "I think it was a mistake to combine styling and layout"

Perhaps because we can have different styles of layout, it fits with CSS better than you say. Carefully designed padding, margins and negative space is both a style and layout consideration.

Containers contain styles, but the container's relationship with other containers may tie with presentation such as border thickness, colour, shadows, and let's not forget animation and interaction effects on containers. Maintaining control of these aspects in one place makes good sense to me.