Downlevel :is

[devongovett]

The :is selector is fairly new so we should probably downlevel it: https://caniuse.com/css-matches-pseudo. We use it currently to compile nesting as well.

In the general case, this is hard if not impossible to do correctly. It can easily explode in combinations and generate massive CSS, and in some cases might be challenging without knowing the DOM because it's essentially boolean logic (if this selector matches, AND this selector matches). But may work in simple cases. See also postcss/postcss-selector-matches#19 (comment) and https://github.com/csstools/postcss-is-pseudo-class.

Perhaps we could compile to the older :matches or :-webkit-any syntax at least.