I think CSS and the domain of laying out things in 2d (and responding to state changes) with code is actually inherently complex, so which ever layout system you use is going to have a steep learning curve.

Many of the newer CSS features prevent you using JavaScript to achieve the same thing, which is better as it results in better performance and easier to understand code.

I would rather have newer standardised selectors than see the same pattern re-implemented with JS differently in every codebase.