Earlier quoted context omitted.
What was your objection to writing CSS in JS? It would seem like a fairly trivial syntactic transition to go from one to the other? It's just selectors with a dictionary of properties right?
Writing CSS in Javascript brings back memories of pre-2000 web development, table based layouts and more horribly inline styles inside HTML tags. CSS was a breath of fresh air, it meant separation of concerns.. HTML the content and CSS the visual representation. With the advent of jQuery it allowed separation of behavior as well. With frameworks like React these days, content+behavior is in JavaScript already, and wi…
When you start thinking in components it becomes much nicer to have the styling, logic and layout all in one file, isolated from the rest of the application and easy to edit, move around and apply to any page.
The previous 'best practice' of having these coupled aspects in separate files now seems quaint and tedious.
There are of course trade-offs, and CSS-in-JS has not triumphed yet, but if you've held off from considering this approach simply because you've had 'separation of technologies' drummed into you in the past you might be missing out.