13. A computer-readable storage medium storing computer-executable instructions that, when executed by a client computer system, causes the client computer system to perform a computer process that presents a customized user interface page in an application, the computer process comprising: receiving an application package from a communications network at the client computer system, the application package containing markup data of the customized user interface page, the markup data defining a user interface element that is potentially available to a user of the client computer system in the customized user interface page and one or more constraints associated with the user interface element for selectively presenting or hiding the user interface element based on properties of the application or the client computer system; searching the markup data of the customized user interface page for the one or more constraints associated with the user interface element, wherein each of the one or more constraints associated with the user interface element in the markup data specifies a condition for presenting the user interface element which is to be evaluated against a configuration parameter that identifies one or more configuration-specific characteristics of the application or the client computer system; compiling the one or more constraints associated with the user interface element into a constraint expression; determining whether the constraint expression is satisfied by evaluating each of the one or more constraints in the constraint expression against the corresponding configuration parameter; rendering the customized user interface page to include the user interface element in a display, if the constraint expression is satisfied; and rendering the customized user interface page to hide or omit the user interface element, if the constraint expression is not satisfied.
14. The computer-readable storage medium of claim 13 wherein the application is executed by the client computer system and the determining operation comprises: evaluating a constraint in the constraint expression against a configuration parameter of the client computer system.