Aspects of the technology described herein detect visible abnormalities within a webpage or other document. The technology improves computing accuracy by identifying data and/or programing errors that cause the abnormalities. The abnormalities are detected through image analysis of portions of a document. Initially, a portion of a webpage associated with a particular feature is identified and then converted to a digital image. The digital image can capture the website as it would appear to a user viewing the website, for example, in a web browser application. The image is then analyzed against an established feature-pattern for the feature to determine whether the image falls outside of a normal range. When the image of a portion of the webpage falls outside of the normal range, a notification can be communicated to a person associated with the webpage, such as a system administrator.