Comment by rhdunn

The question is really about where the boundary between presentation (CSS) and interactivity (JavaScript) lies.

For static content like documents the distinction is easy to determine. When you think about applications, widgets, and other interactive elements the line starts to blur.

Before things like flex layout, positioning content with a 100% height was hard, resulting in JavaScript being used for layout and positioning.

Positioning a dropdown menu, tooltip, or other content required JavaScript. Now you can specify the anchor position of the element via CSS properties. Determining which anchor position to use also required JavaScript, but with things like if() can now be done directly in CSS.

Implementing disclosure elements had to be done with a mix of JavaScript and CSS. Now you can use the details/summary elements and CSS to style the open/close states.

Animation effects when opening an element, on hover, etc. such as easing in colour transitions can easily be done in CSS now. Plus, with the reduced motion media query you can gate those effects to that user preference in CSS.