Original issue cssnano/cssnano#645

This valid css

--aspect-ratio: 16/9;
padding-top: calc(100% / (var(--aspect-ratio)));

becomes invalid after cssnano compilation, because cssnano removes parenthesis around of var()

Chrome and Firefox don't respect the var() without parenthesis around it and so the code is not working as expected.

Test case: https://jsfiddle.net/z7bsn1r8/4/