Servo uses Taffy for CSS Grid. It could also very easily use it for Flexbox, but they currently prefer to use their own implementation there. It was originally

Servo uses Taffy for CSS Grid. It could also very easily use it for Flexbox, but they currently prefer to use their own implementation there. It was originally a derivative of React Native's Yoga implementation of Flexbox, and is currently developed primarily as part of the Blitz engine.