code: function loadCss(url, ignoreDuplicate, cb) {
          if (ignoreDuplicate && loadedRes[url]) {
              cb && cb();
              return;
          }
          if (ignoreDuplicate && loadingRes[url]) {
              loadingRes[url].push(cb);
              return;
          }
          loadingRes[url] = loadingRes[url] || [];
          var link = d.createElement('link');
          link.type = 'text/css';
          link.rel = 'stylesheet';
          link.href = url;
  
          if (browser === 'msie') {
              link.onreadystatechange = function () {
                  /loaded|complete/.test(link.readyState) && cb();
              }
          }
          else if (browser === 'opera') {
              link.onload = cb;
          }
          else {
              // FF, Safari, Chrome
              (function () {
                  try {
                      link.sheet.cssRule;
                  }
                  catch (e) {
                      setTimeout(arguments.callee, 20);
                      return;
                  }
                  loadedRes[url] = true;
                  if (ignoreDuplicate) {
                      for (var i = 0; i < loadingRes[url].length; i++) {
                          loadingRes[url][i] && loadingRes[url][i]();
                      }
                      loadingRes[url] = null;
                  }
                  cb();
              })();
          }
  
          head.appendChild(link);
      }
docs: load css and apply it. 加载 css, 并应用该样式。