1. An integrated development environment for developing user interface documents, the integrated development environment comprising: a processor of a machine; an editor to edit a user interface document; an adaptation engine to generate device class specific representations of the user interface document, each device class specific representation referring to a respective device class; and a device class dependent complexity indicator to determine, using the processor of the machine, complexity values of layout components of the device class specific representations by accessing a library of stored complexity evaluation functions associated with the layout components, each combination of a layout component with respect to each device class having a distinct complexity evaluation function in the library, to calculate a complexity value for each of the layout components with respect to each of the respective device classes in the device class specific representations using the corresponding distinct complexity evaluation function accessed from the library, and to aggregate the complexity values of the respective device class specific representations into a single complexity value for each device class, the complexity values being numerical values.