docs: function updateCSSContentUrl(content, file, pkg, context) {
      return fis.compile.extCss(content, function (m, comment, url, last, filter) {
          url || (url = filter);
          if (url) {
              var leftQuot = '';
              var rightQuot = '';
              var processUrl = url.replace(/^('|")([^'"]+)('|")$/,
                  function (all, lquot, uri, rquot) {
                      leftQuot = lquot || '';
                      rightQuot = rquot || '';
                      return uri;
                  }
              );
              var uri = context.resolvePath(processUrl, file);
  
              if (uri === processUrl) {
                  return m;
              }
  
              var msg = {
                  target: uri,
                  file: pkg,
                  ret: uri
              };
  
              /**
               * @event plugin:relative:fetch 获取相对路径的事件
               */
              fis.emit('plugin:relative:fetch', msg);
  
              m = m.replace(url, leftQuot + msg.ret + rightQuot);
          }
  
          return m;
      }, file);
  }