code: function (rule) {
  
          var that = this;
          var base = this.base;
          var options = this.options;
          
          var url = utils.unquotation(rule.href.trim());
          url = utils.resolve(base, url);
          url = this._getUrl(url);
  
  
          if (!url) {
              return;
          }
  
  
          this.importLength ++;   
  
          // 限制导入的样式数量，避免让爬虫进入死循环陷阱
          if (this.importLength > options.maxImportCss) {
              var errors = new Error('the number of files imported exceeds the maximum limit');
              errors = new VError(errors, 'parse "%s" failed', that.file);
              return Promise.reject(errors);
          }
  
  
          var resource = new Resource(url, null, options)
          .catch(function (errors) {
              errors = new VError(errors, 'parse "%s" failed', that.file);
              return Promise.reject(errors);
          });
  
  
          return new CssParser(resource, this.importLength);
      }
docs: CSS 导入规则 @import url("fineprint.css") print; @import url("bluish.css") projection, tv; @import 'custom.css'; @import url("chrome://communicator/skin/"); @import "common.css" screen, projection; @import url('landscape.css') screen and (orientation:landscape);