I don't think it's that clear cut, you can define all sorts of behavior with both HTML and CSS, and there's no semantic meaning in `<div>`, which itself changes

I don't think it's that clear cut, you can define all sorts of behavior with both HTML and CSS, and there's no semantic meaning in `<div>`, which itself changes page style due to its layout properties. I don't know what goalposts are being moved, React/css-in-js have never violated separation of concerns. After all, css-in-js produces CSS, and React produces HTML. Separation of technologies isn't a principle, but DSLs are a well established pattern.