That philosophy works if you have a lot of docs with uniform content that can be represented by simple tags that don't need to change much. However if you have

That philosophy works if you have a lot of docs with uniform content that can be represented by simple tags that don't need to change much. However if you have a rich UI then A) there tends to be more structure than content and B) structure, behavior and presentation are coupled together by definition. Yes you can try to shove the square post into a round hole by naming your HTML just-so, cramming as much presentation in your CSS with pseudo elements, and using pure JS files hooked to HTML via classes/ids, but the minute you need to change anything you'll see you can't make a single change without carefully considering all 3 "de-coupled" file types. Worse, if you try to reuse these semantic definitions across a large app, now you find that a change over here can break things way over there, the larger the app the worse it gets. Once you get to this point you realize that component level isolation ala React/JSX is a much more sane approach to manage huge web app UIs than pretending that functional de-coupling is some kind of platonic ideal rather than just one possible set of tradeoffs.