index.js

var glob = require("glob");
var fs = require("fs");

var optimizers = {
    "clean-css": function (content) {
        var optimizer = require("clean-css");

        return new optimizer({}).minify(content);
    },
    "csso": function (content) {
        var optimizer = require("csso");

        return optimizer.justDoIt(content);
    }
};

glob("input/*.css", function (error, files) {
    files.forEach(function (file) {
        console.log("# " + file);

        var content = fs.readFileSync(file).toString();
        console.log("raw: " + content.length);

        for (var identifier in optimizers) {
            var optimizer = optimizers[identifier];

            var minimized = optimizer(content);
            console.log(identifier + ": " + minimized.length);
        }
    });
});

package.json

{
  "name": "css-benchmark",
  "version": "1.0.0",
  "author": "eiszfuchs",
  "license": "WTFPL",
  "dependencies": {
    "glob": "^4.0.6",
    
    "clean-css": "^2.2.16",
    "csso": "^1.3.11"
  }
}