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);
}