code: function getHTML(files, options) {
      if (_.isString(files)) {
          files = [files];
      }
  
      files = _.flatten(files.map((file) => {
          if (!isURL(file) && !isHTML(file)) {
              return glob.sync(file);
          }
          return file;
      }));
  
      if (!files.length) {
          return Promise.reject(new Error('UnCSS: no HTML files found'));
      }
  
      // Save files for later reference.
      options.files = files;
      return Promise.all(files.map((file) => jsdom.fromSource(file, options)));
  }
docs: Get the contents of HTML pages through jsdom.
  @param  {Array}   files   List of HTML files
  @param  {Object}  options UnCSS options
  @return {Array|Promise}