If you are using something like uMatrix you might have noticed that theres a script element trying to load. You can safely permit it since it only preloads deferenced css. It is a technique that I have not taken a look into very deeply, but it does make this website 20% faster, so that is sufficient for me.
If you want to implement this on your site, all you have to do is check which part of your CSS stylesheet is critical to load with your browser’s dev tool, and add it to the head section of your site. View this website’s code for a reference.