"Aggressive" merging fails when merging an rules with an "invalid" pseudo class rule

[stephenrose]

Duplicated from my ticket here: frontendfriends/grunt-combine-mq#14

As invalid rules are ignored, .grey will be ignored by browsers that are not webkit, and don't support ::-webkit-*

I see a solution being to blacklist rules that are "invalid" (for any reason), so they are not merged with any other rules.

Input CSS

::-webkit-input-placeholder{
  color:grey;
}


.box{
color:grey;
}

Failing output CSS

::-webkit-input-placeholder, .box{
  color:grey;
}