SXA provides the ability for optimizing CSS styles and JavaScript via SXA Asset Optimizer. If you want to know more about what the Asset Optimizer is you can check out here.

However,  we found a bug in the Asset Optimizer, which stripes out the key/reserved words i.e. (switch, for, imports etc..) in CSS styles during minifying in all the versions of SXA, apart from SXA 1.6.

Given the complexity for fixing this issue, Stiecore support suggested to either disable the optimizer or upgrade to SXA 1.6, if you are facing the same issue.