<style type="text/css">
*{margin:0px;padding:0px;font-size:12px;}
input{width:100px;height:20px;border:1px solid #ccc;}
</style>

<script language="javascript">
function tips(id,str) {
var l=document.getElementById(id).offsetLeft+120;
var t=document.getElementById(id).offsetTop;
document.getElementById("tips").innerHTML="提示："+str;
document.getElementById("tips").style.left=l+"px";
document.getElementById("tips").style.top=t+"px";
document.getElementById("tips").style.display="";
}
function outtips(){
 document.getElementById("tips").style.display='none';
}
</script>
<div id="tips" style="position:absolute;border:1px solid #ccc;padding:0px 3px;color:#f00;display:none;height:20px;line-height:20px;background:#fcfcfc"></div>
<br>
姓名：<input type="text" id="username" onfocus="tips('username','姓名长度最多16个字符')" onblur="outtips()">
<br>
密码：<input type="password" id="password" onfocus="tips('password','密码长度必须在3-18位之间')" onblur="outtips()">