Is CSSOM and DOM creation asynchronous?

I have read that CSSOM creation is a bottleneck in terms of web page performance. But there seems to be some ways around it, like adding the media property to the stylesheet link. I'm trying to understand how to optimise my web app and came across this really interesting link but couldn't understand what order CSSOM and DOM creation happen in.

Here I see some reference to asynchronous loading of CSS files, but the answer is not very clear. Of course, that is about loading and not object model creation.

My question is this: Does the CSSOM creation and DOM creation happen in parallel or in sequence?