code: function styleSheetToString(styleSheet) {
      var result = "";
      var rules = styleSheet.cssRules,
          rule;
      for (var i = 0; i < rules.length; i++) {
          rule = rules[i];
          if (rule instanceof CSSOM.CSSImportRule) {
              result += styleSheetToString(rule.styleSheet) + '\n';
          } else {
              result += rule.cssText + '\n';
          }
      }
      return result;
  }
docs: 把 StyleSheet 的内容转换成 css 字符串
  @param  {StyleSheet} styleSheet
  @return {String} css 字符串