If you really want to get fancy you could use Selenium to get screenshots and compare them to check that the remynified CSS produces the same layout as the original CSS.
Note that perceptual hashes should not obey the avalanche effect, so a minor difference in the two images should yield a small change in the hashes (desirable in this case). Then accept results that are within some small % error margin.
An example of a library that does this is jimp [1].
[0] https://en.wikipedia.org/wiki/Perceptual_hashing [1] https://github.com/oliver-moran/jimp#comparing-images
Especially if media queries come into play and we'd have to try different resolutions and ensure they're all the same.
Or even more simply: if your CSS is supposed to work on all pages of a website.