Most articles about CSS refactoring focus on specific refactoring techniques. They rarely even mention testing as a safety net for changes. Those that do describe ad-hoc manual testing or visual regression testing using image diff tools such as Wraith, PhantomCSS or BackstopJS. These tools take a little setting up, but they can quickly tell you if a page or component differs from an expected ‘reference’ image. You can use visual regression tests if you want to refactor code to produce the same result that you already have.
Are you writing legacy CSS code? | Tinned Fruit