1. A method of presenting a customized user interface page in an application, the method comprising: receiving an application package from a communications network at a 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.
11. The method of claim 1 wherein the application is executed by the client computer system and the determining operation comprises: evaluating a plurality of constraints in the constraint expression against a plurality of configuration parameters identifying properties of the application or the client computer system; and determining whether a Boolean expression combining the plurality of constraints is satisfied.