Diffing is similar to eyeballing the content but the comparison is executed programmatically, so if you want to use it as a testing method, you need to provide the diff tool with a copy of the output expected (a control document) as well as the content to check.

This technique is especially useful if you don’t have a schema to validate against and also for spotting lost or unintentionally changed content. However, if the control document is large, it can be very time-consuming to create and maintain.