Project Quantum is a major rewrite of Firefox’s internals to make Firefox fast. We’re swapping in parts from our experimental browser, Servo, and making massive improvements to other parts of the engine.

The first major component from Servo—a new CSS engine called Quantum CSS (previously known as Stylo)—is now available for testing in our Nightly version. It brings together state-of-the-art innovations from four different browsers to create a new super CSS engine.