Sometimes the most important quality check that you can make is to ensure that a change that you made does not have any unintended consequences. Regression testing is as important in document formatting as it is in any other software field.

When the markup is deeply nested and contains multiple interacting properties, or the properties are defined in separate stylesheets containing rules that cascade in ways that may not be immediately obvious, then checking the formatted output for changes can be better use of your time than looking at diffs of files and trying to imagine the effects.

Antenna House developed a visual regression testing system for its own use that was then released as a product. It can compare single raster images, multi-page documents, or directories of documents and produce both summary reports and reports showing the exact visual differences between two pages. The system rasterises its source documents at a user-defined resolution and compares them pixel-by-pixel. As such, the sources do not have to be from Antenna House Formatter and, in fact, could be generated by two disparate sources.