1. One or more memories having computer-readable instructions thereon which, when executed by a computer, cause the computer to perform acts comprising: providing multiple skin elements that constitute part of a user interface, the multiple skin elements comprising at least a first skin element and a second skin element; and synchronizing a property of the first skin element with a property of the second skin element, the synchronizing comprising describing a synchronization between the property of the first skin element and the property of the second skin element using a tree data structure having multiple nodes, a first node of the tree data structure representing a visible region of the multiple skin elements and a second node of the tree data structure representing an invisible region of the multiple skin elements, wherein one of the first skin element and the second skin element is associated with the first node, and the other of the first skin element is associated with the second node.