==== Quantum CSS (Stylo) ====
Starting with Firefox 57 and later, Gecko makes use of the parallel style system written in Rust that comes from Servo. There's an [https://hacks.mozilla.org/2017/08/inside-a-super-fast-css-engine-quantum-css-aka-stylo/ overview] of this with graphics to help explain what's going on. The [https://github.com/servo/servo/wiki/Layout-Overview Servo wiki] has some more details.
==== Gecko ====