Disclosed herein are technologies for a WYSIWYG user interface (UI) design tool that enables runtime behaviors (e.g., content update and user interaction) with UI controls during design time of such controls. That is, using a UI design tool in accordance with the technology described herein, the human designer can see and experience the runtime behavior of the UI controls while designing those controls. This Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.