1. A method of testing visual elements in a rendered web page, comprising: defining a gold image at a first point within a web application; taking a screen shot of an actual image at the first point during execution of the web application; and comparing the gold image to the actual image and generating a difference image based on the comparison; wherein the difference image comprises: a first region highlighting a first difference between the gold image and the actual image within an area common to both images; and a second region highlighting a second difference between the gold image and the actual image which is not within an area common to both images.