CSS is (too) powerful but not inherently complex. It is often abused.
Ultimately you will need real eyeballs to judge aesthetics. No argument there. But significant QA work can be automated with frontend testing frameworks.
I've never been that impressed with the functional coverage you get from front-end-driven testing frameworks, at least those that drive views. Test that interface with models in the front-end, though -- priceless!
And CSS is inherently complex, because the rules of precedence are fragile and easily broken.
Ultimately you will need real eyeballs to judge aesthetics. No argument there. But significant QA work can be automated with frontend testing frameworks.