function combineCss(cssArray) {
  if (cssArray.length === 1) {
    return cssArray[0].toString();
  }

  return new CleanCSS().minify(invokeMap(cssArray, 'toString').join(' ')).styles;
}
Returns a string of combined and deduped css rules.
@param {array} cssArray Array with css strings
@returns {String} combined and deduped css rules