动态为页面添加CSS样式文件引用

动态为页面添加CSS样式文件引用：

 if (document.createStyleSheet) { //IE
     document.createStyleSheet("./Themes/Default/MessageTip.css");
 }
 else { //Firefox, Chrome
     var stylesheet = document.createElement("link");
     stylesheet.href = "./Themes/Default/MessageTip.css";
     stylesheet.rel = "stylesheet";
     stylesheet.type = "text/css";
     document.getElementsByTagName("head")[0].appendChild(stylesheet);
 }