A non-tree representation of a UI control tree is provided by a compact UI binary file that is generated by encoding a UI definition markup file expressing UI controls and behavior in human-readable form. The UI binary file is utilized in a runtime environment on a computing device as a source of a binary instruction stream. The stream can be efficiently processed by an interpreter on the device without needing validation between loading and rendering the UI. The encoding places much of the representation into an object and script section of the UI binary file. The interpreter runs this section without it being entirely resident in the device's memory to minimize the memory footprint. At runtime, operation code (“op-code”) streams contained in this section are used to build UI objects, and implement scriptable behavior for manipulating the UI objects to render the UI on the device with the designed behavior.