The complexity of data-intensive business processes is typically reflected in the user interfaces of the information systems supporting them. To facilitate ease of use despite the visual and semantic complexity of dialog forms, users should be guided in ways such as highlighting and describing invalid input, showing/hiding or enabling/disabling particular UI widgets. Manual implementation of the rules governing these user interface responses typically requires considerable effort, as they involve business and presentation concerns, and are often dependent on each other. In addition, users expect the interface to respond immediately to any input, which can be especially challenging in web applications. In this paper, we formalize the aspects that must be considered in formulating input evaluation rules, and present a run-time framework that encapsulates the necessary logic, enabling domain experts to specify the business rules instead of requiring developers to implement them.