code: function getCSS([files, options, pages, stylesheets]) {
      /* Ignore specified stylesheets */
      if (options.ignoreSheets.length) {
          stylesheets = stylesheets
          .map((arr) => {
              return arr.filter((sheet) => {
                  return _.every(options.ignoreSheets, (ignore) => {
                      if (_.isRegExp(ignore)) {
                          return !ignore.test(sheet);
                      }
                      return sheet !== ignore;
                  });
              });
          });
      }
  
      if (_.flatten(stylesheets).length) {
          /* Only run this if we found links to stylesheets (there may be none...)
           *  files       = ['some_file.html', 'some_other_file.html']
           *  stylesheets = [['relative_css_path.css', ...],
           *                 ['maybe_a_duplicate.css', ...]]
           * We need to - make the stylesheets' paths relative to the HTML files,
           *            - flatten the array,
           *            - remove duplicates
           */
          stylesheets =
              _.chain(stylesheets)
                  .map((sheets, i) => utility.parsePaths(files[i], sheets, options))
                  .flatten()
                  .uniq()
                  .value();
      } else {
          /* Reset the array if we didn't find any link tags */
          stylesheets = [];
      }
      return Promise.all([options, pages, utility.readStylesheets(stylesheets, options.banner)]);
  }
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
  @param  {Array}   stylesheets List of CSS files
  @return {Array}