Despite having good unit test coverage and automated end-to-end functional tests, a full manual regression of the UI, covering multiple operating systems, web browsers, screen resolutions, and localizations, took days to complete.
Back at the time, there were plenty of excellent commercial and open-source tools that allowed you to test the functionality of your app through the UI, but there were no tools that allowed you to automatically test the look & feel and user experience of your app. That is, verifying that each UI element in each page appears in the right color, shape, position, and size, and that it does not overlap or hide other UI elements.