https://drafts.csswg.org/selectors-4/#matches

Pseudo-elements cannot be represented by the matches-any pseudo-class; they are not valid within :matches().

If I'm understanding this correctly, pseudo-elements shouldn't be allowed inside :matches per specification.

I tested this code:

.test :matches(input, ::placeholder) { ... }

And it will produce this result:

.test input, .test ::placeholder { ... }

Not sure if the intention is to follow specification as closely as possible. If it is, I'm guessing we could check for appearance of :: in bodySelectors array of explodeSelector function and then throw a warning/error, maybe even just return [selector] silently? We could even check against an array of pseudo-elements (there aren't many of them) in case someone uses single : to express pseudo-element.

Any thoughts?

I’m all for following the spec. I think this plugin needs to be deprecated. Otherwise, people will demand that this plugin support both the old pattern and the new pattern. It would be better if we let this plugin do what it has always done (and allow people to explicitly enable it in the next major release of PostCSS Preset Env), and then also create a new plugin for :is which follows the spec.