code: function getStylesheets(files, options, pages) {
      if (options.stylesheets && options.stylesheets.length) {
          /* Simulate the behavior below */
          return Promise.resolve([files, options, pages, [options.stylesheets]]);
      }
      /* Extract the stylesheets from the HTML */
      return Promise.all(pages.map((page) => jsdom.getStylesheets(page, options)))
      .then((stylesheets) => [files, options, pages, stylesheets]);
  }
docs: Get the contents of CSS files.
  @param  {Array}   files   List of HTML files
  @param  {Object}  options UnCSS options
  @param  {Array}   pages   Pages opened by jsdom
  @return {Promise}