You might do a little reconsidering about what the purpose of validation is. It’s not an end in itself, and if your website functions perfectly without validating, and still has semantically meaningful markup, then validation is unnecessary. As Jeff Croft said, “Your markup validator, whether it’s the one on the W3C site or one built into your favorite coding tool, is a debugging tool [...] not a measuring stick for greatness.” I’m sure that you’ve heard about not fixing things that aren’t broken?
Your markup validator.
Or to cite an earlier article on a similar idea March to Your Own Standard
Also, there’s no way CleanCSS magically fixed your validation problem, unless it took the opacity declaration out. Check your code.