1. A method performed by a data processing apparatus, the method comprising: receiving input specifying a change to a parameter relating to display of a visual content in a webpage; in response to receiving the input, generating a test image of the webpage including the visual content, wherein the test image of the webpage represents a visual appearance of the webpage as displayed to a user of a client device; comparing the generated test image with a predetermined target image of the visual content to determine whether the visual content included in the generated test image is visually consistent with the predetermined target image of the visual content, wherein the predetermined target image of the visual content represents an expected visual appearance of the visual content as displayed to the user of the client device, and the predetermined target image is stored in a repository; and selectively providing a failure notification based upon a result of the comparison; wherein comparing the generated test image with the predetermined target image of the visual content comprises: selecting a plurality of different portions of the generated test image, where each portion has a same size as the predetermined target image, determining, for each portion, a difference for the portion by comparing pixels of the portion with pixels of the predetermined target image, identifying a portion of the plurality of different portions having a smallest difference, and determining whether the smallest difference of the portion is smaller than a threshold difference; wherein the visual content included in the generated test image of the webpage is determined to be visually consistent with the predetermined target image of the visual content when the smallest difference for the portion is smaller than the threshold difference.