为什么用本地浏览器关闭页面时没有提醒确定离开
<!DOCTYPE html>
<html>
<meta charset="utf-8">
<head>
<style type="text/css">
div#con1{height:200px;width:300px;background-color: #eee8aa;margin-left: 25%;margin-top:5%;}
div#con2{height:400px;width:600px;background-color: #eee8aa;margin-left:25%;margin-top:5%;}
body{background:url("img/fp.jpg");background-size: cover;}
</style>
<script type="text/javascript">
window.onbeforeunload = unloadMessage;
function unloadMessage(){
return "确定关闭吗？";
}
</script>
</head>
<body>
<p align=center><font size=15px color= #eee8aa><i>欢迎访问，请先登录！！！<i></font></p>
<div id=con2>
<table border="0" bgcolor=black height=60px width=600px>
<tr>
<td><font size=6 color=#ffffff>请登录：</font>
</td>
</tr>
</table><br><br><br>
<div id=con1>
<form>
用户名：
<input type="text" name="user" style="width:200px;height:30px">
<br><br>
密码：
<input type="password" name="passsword"style="width:200px;height:30px">
<p>
<font size=2 color=red><a href="zhuce.html" target="_blank" style="text-decoration:none"> 没有账号？请注册</a></font></p>
<a href="g.html" target="_blank" style="text-decoration:none"><button type="button" >登录</button></a>
<a href="zhuce.html" target="_blank" style="text-decoration:none"> <button type="button" margin-left=50px>注册</button></a>
<a href="g.html" target="_blank" style="text-decoration:none"> <button type="button"style="width:100px;height:30px" >返回主页</button></a>
</div>
</div>
</html>
