13. A system for displaying a user interface of a computer program, the computer program having a plurality of different functions that are selectable by an end user, the system comprising: a processor; a display; computer-readable storage media accessible by the processor, the computer-readable storage media having stored thereon: a plug-in module configured to implement at least one function of the computer program; a first mark-up language document provided by the plug-in module, the first mark-up language document comprising data representing functionality available to the end user based on the plug-in module, wherein the first document specifies a functionality of menu items that are to be displayed; a second mark-up language document provided by the plug-in module, comprising a style-sheet language document, the style-sheet language document comprising data representing a display style that is associated with the plug-in module, wherein the style-sheet language document is editable by the end user, including subsequent to implementation of the style sheet language document, to change the appearance of the image that is rendered by the program; and a third mark-up language document created by the system, the third mark-up language document created based on processing the first mark-up language document with the second mark-up language document, the third mark-up language document representing both the functionality and the display style associated with the plug-in module, the user interface being implemented based on the third mark-up language document and being responsive to markers within the third document that determine which one of a plurality of windows is triggered when a hyperlink is selected by an end user.
13. A system for displaying a user interface of a computer program, the computer program having a plurality of different functions that are selectable by an end user, the system comprising: a processor; a display; computer-readable storage media accessible by the processor, the computer-readable storage media having stored thereon: a plug-in module configured to implement at least one function of the computer program; a first mark-up language document provided by the plug-in module, the first mark-up language document comprising data representing functionality available to the end user based on the plug-in module, wherein the first document specifies a functionality of menu items that are to be displayed; a second mark-up language document provided by the plug-in module, comprising a style-sheet language document, the style-sheet language document comprising data representing a display style that is associated with the plug-in module, wherein the style-sheet language document is editable by the end user, including subsequent to implementation of the style sheet language document, to change the appearance of the image that is rendered by the program; and a third mark-up language document created by the system, the third mark-up language document created based on processing the first mark-up language document with the second mark-up language document, the third mark-up language document representing both the functionality and the display style associated with the plug-in module, the user interface being implemented based on the third mark-up language document and being responsive to markers within the third document that determine which one of a plurality of windows is triggered when a hyperlink is selected by an end user. 14. The system of claim 13 , wherein the first mark-up language document is an extensible mark-up language document, and the third mark-up language document is a hypertext mark-up language document.