The W3C standard API for determining the computed style of an element is the getComputedStyle() method of the Window object. The first argument to this method is the element whose computed style is desired. The second argument is any CSS pseudoelement, such as ":before" or ":after" whose style is desired. In the Mozilla and Firefox implementation of this method, the second argument is required and may not be omitted. As a result, you'll usually see getComputedStyle() invoked with null as its second argument.