code: function getUsedSelectors(page, css) {
      let usedSelectors = [];
      css.walkRules((rule) => {
          usedSelectors = _.concat(usedSelectors, rule.selectors.map(dePseudify));
      });
      return jsdom.findAll(page, usedSelectors);
  }
docs: Find which selectors are used in {pages}
  @param  {Array}    page          List of jsdom pages
  @param  {Object}   css           The postcss.Root node
  @return {Promise}