An application can be run and initial user interface definitions for the application can be loaded into memory. The application can use the initial user interface definitions in the memory to present an initial user interface for the application. In response to user input, one or more updates can be made to the initial user interface definitions in the memory, and the application can use the updated user interface definitions to present an updated user interface for the application. Presenting the initial user interface, making the updates, and using the updated definitions to present the updated user interface can be done while the application continues to run.