CSS like any other source code needs refactoring. We work to improve code readability and reusability, to make CSS faster to execute. We try alternative solutions (like switching from flexbox to grid). As we are done with the code changes we need to ensure the app’s “look & feel” didn’t alter. It can turn out quite a challenge if do it manually – the app may consist of dozens of pages that need to be tested for every defined viewport breakpoint. Here you can take advantage of Puppetry. All what you need to do is to create tests visiting app pages for given breakpoint viewports and call page.assertScreenshot or TARGET.assertScreenshot method.