代码如下：
```html
<script type="text/javascript">
var flag =false;
function check(){
if(form1.username.value==""){
alert("用户名为空，请重新登录！");
return false;}
if(form1.pwd.value==""){
alert("密码为空，请重新登录！");
return false;}
url='cheackDl.php?username='+form1.username.value+'&pwd='+form1.pwd.value;
http_request = new XMLHttpRequest();
http_request.onreadystatechange = alertContents;
http_request.open('GET', url, false);
http_request.send();
return flag;
}
function alertContents(){
if(http_request.readyState==4){
if(http_request.status==200){
if(http_request.responseText==1){
alert("用户名错误！");}
if(http_request.responseText==2){
alert("密码错误！");}
if(http_request.responseText==3){
alert("成功登录");
flag=true;}
}
}
}
</script>
<html>
<div align="center" onsubmit="return check()">
<p>登录页面</p>
<form action="zhy.php" method="get" name="form1" id="form1"> <!-- 表单 -->
<input type="text" name="username" id="username"> <!-- 用户名输入框 -->
<br>
<input type="password" name="pwd" id="pwd" /> <!-- 密码输入框 -->
<br>
<input type="submit" name="userzc" id="userzc" "> <!-- 提交按钮 -->
</form>
</div>
</html>
```