1. A method comprising: determining an animation for a target graphical user interface (GUI) element; setting an input of the animation to relate to a property of a source GUI element, wherein a key frame of the animation specifies, for a point at the key frame of the animation, a specific value of a property of the target GUI element at a specific value of the input of the animation set to relate to the property of the source GUI element; and with a computing device that implements a rendering engine, at runtime, rendering for display the target GUI element according to the animation and runtime value of the property of the source GUI element, wherein the rendering includes iteratively: determining the runtime value of the property of the source GUI element; normalizing the runtime value of the property of the source GUI element according to length of the animation; and interpolating within the animation to compute a runtime value of the property of the target GUI element at the normalized runtime value of the property of the source GUI element.