1. A computer-readable medium having computer-executable instructions for performing a method of using expressions to establish a relationship between properties in code executable by a browser running on a computer, each property defining a characteristic of an object, wherein the computer-readable instructions are written in HTML code and define a document that forms the browser, the method comprising: declaring the value of a first property as a scalar value, wherein the first property is a width of a window of the browser; declaring the value of a second property as an expression, wherein the second property is a function of the first property, and wherein the expression represents an element in the window of the browser; automatically changing the value of the second property in response to a change in the value of the first property; and updating the document upon changing the value of the second property.