Yoga
Cross-platform layout engine which implements Flexbox
...Yoga has unlocked exciting features such as calculating layouts off of the main thread to help ensure smooth UI performance. Yoga was built to be fast and performance will always be one of Yoga's primary goals. For a layout engine to be able to power any range of applications, it needs to be fast and never stand in the way of a fluid user experience. Yoga is built with cross platform in mind. To ensure Yoga can be used anywhere, it was written in portable C/C++ and has a low number of dependencies and small binary size. This means Yoga can be used on iOS and Android, sharing knowledge, and potentially code, between platforms.