code: function uniqueCss(css) {
      if (!css) {
          return '';
      }
  
      css = new CleanCSS({
          compatibility: 'ie9',
          format: {
              breaks: {afterAtRule: true, afterRuleEnds: true}
          }
      }).minify(css).styles;
  
      let array = css.split('\n');
      let res = [];
      for (let i = 0, len = array.length; i < len; i++) {
          let current = array[i];
          if (res.indexOf(current) === -1) {
              res.push(current);
          }
      }
      return res.join('\n');
  }
docs: /*
  remove duplicate css