Agreed. I think part of why it's hard to convey is that a lot of people formed their idea of CSS back when flexbox was new. These days an accordion is a few <details>, and a reading progress bar is a fixed div with animation-timeline: scroll(). You don't need JS for either.
(Full disclosure, I wrote a book on this, so I'm not exactly neutral: https://theosoti.com/you-dont-need-js/)