Calculations are performed relative to a fixed time interval so that extreme variations in the frame-rate shouldn’t cause the physics to explode. The algorithm (integrator) that’s used to determine the motion of objects can be customised too, keeping things nice and flexible.
3D support is said to be in the pipeline, and there’s apparently still room for optimisation; cool!
Six demos are available, including: attraction, chain, cloth and balloon – all of which tick along at a solid 50+ FPS. For best results use Chrome, work is underway to improve stability/performance in other browsers.