Once sorted, the essential (or eCSSential if you will) files are loaded in a way that ensures page rendering will be blocked until they're ready. The other less-essential files are loaded in a non-blocking way, letting the page render while they are fetched.
In one further improvement to browsers' default loading behavior, stylesheets that are loaded in a blocking manner are given 8 seconds (by default) to load before they are refetched asynchonously, allowing the page to appear and be used.
Demo: https:/ / github. com/ scottjehl/ eCSSential#check-out-the-demos
License: MIT License or GPL