- A browser can’t render a page until it has built the Render Tree;
- the Render Tree is the combined result of the DOM and the CSSOM;
- the CSSOM is all CSS rules applied against the DOM;
- making CSS asynchronous is much more difficult;
- so a good rule of thumb to remember is that your page will only render as quickly as your slowest stylesheet.
There are lots of options to do better with this, including some interesting things that HTTP/2 unlocks.
Check out Šime Vidas's takeaways as well. It's all fascinating, but the progressive rendering stuff is particularly cool. I suspect many CSS-in-JS libraries could/should help with doing things this way.