Rule merging problematic with unsupported selectors/pseudoclasses

[mischnic]

.form-input:-moz-placeholder-shown {
  color: black;
}
.form-input:-ms-input-placeholder {
  color: black;
}
.form-input:placeholder-shown {
  color: black;
}

is merged into a single rule. But browsers ignore rules with invalid selectors, so the rule won't apply on any browser.

via https://github.com/evanw/esbuild/releases/tag/v0.13.14

[devongovett]

Yeah. Need to handle vendor prefixes for pseudo selectors the same way we handle them for declarations as well, e.g. adding and removing according to browser targets.

[devongovett]

Should be fixed by f1a074f. Now it checks that all of the selectors in both rules are supported by all targets before merging rules.