In many cases that division doesn't exist. Part of the beauty of React, as far as I'm concerned, is that it allows us to think about things on the page in terms of components, rather than as separate blocks of html, and javascript.

It does make sense to bring css into the mix as well, as css can have functional effects. If you have a React component, for example, which explicitly toggles whether items are displayed or not, it makes sense to tie the toggling of the css display property directly into the component, rather than factoring it out into a separate stylesheet and creating an additional dependency.

Note that I'm not suggesting that all styling should be done this way - I think that would be disastrous - but that there is merit in composing the html, css and javascript together.