With frameworks like React these days, content+behavior is in JavaScript already, and with writing CSS in JavaScript, we've gone back 1.5 decades I think.
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.
http://javascriptplayground.com/blog/2016/07/css-modules-web...