title: How to split css and load them only when needed on React+ReactRouter web app (built by WebPack)
body: <p>I have a web app built by Web app, which takes a lot of time to first loading.
  After investigation- I've tried a lot of improvements, also that one of using only critical CSS on server side rendering.
  but now' after SSR has finished- React generates a lot more CSS rules- which are not used at this point,
  so my question is:
  Is that possible to split CSS and load them on demand (i.e.: on route change)?</p>
  
  <p>Any help will be appreciated.
  Thanks in advance!</p>