postcss-selector-not 
PostCSS plugin to transform
:not()W3C CSS leve 4 pseudo class to :not() CSS level 3 selectors
http://dev.w3.org/csswg/selectors-4/#negation
Installation
$ npm install postcss-selector-not
Usage
var postcss = require("postcss")
var output = postcss()
.use(require("postcss-selector-not"))
.process(require("fs").readFileSync("input.css", "utf8"))
.css
Using this input.css:
p:not(:first-child, .special) {
color: red;
}
you will get:
p:not(:first-child):not(.special) {
color: red;
}