code: function filterUnusedSelectors(selectors, ignore, usedSelectors) {
      /* There are some selectors not supported for matching, like
       *   :before, :after
       * They should be removed only if the parent is not found.
       * Example: '.clearfix:before' should be removed only if there
       *          is no '.clearfix'
       */
      return selectors.filter((selector) => {
          selector = dePseudify(selector);
          /* TODO: process @-rules */
          if (selector[0] === '@') {
              return true;
          }
          for (let i = 0, len = ignore.length; i < len; ++i) {
              if (_.isRegExp(ignore[i]) && ignore[i].test(selector)) {
                  return true;
              }
              if (ignore[i] === selector) {
                  return true;
              }
          }
          return usedSelectors.indexOf(selector) !== -1;
      });
  }
docs: Private function used in filterUnusedRules.
  @param  {Array} selectors      CSS selectors created by the CSS parser
  @param  {Array} ignore         List of selectors to be ignored
  @param  {Array} usedSelectors  List of Selectors found in the jsdom pages
  @return {Array}                The selectors matched in the DOMs