<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>a nav</title>
<style type="text/css">
* {
margin: 0;
padding: 0;
font-style: normal;
font-family: 宋体;
}
body {
text-align: center;
font-size: 14px;
}
#content {
margin: 0 auto;
600px;
}
#content #nav {
background: #006400;
height: 32px;
margin-top: 60px;
}
#content #nav ul {
list-style: none;
}
#content #nav ul li {
float: left;
100px;
line-height: 32px;
position: relative;
}
#nav div {
100px;
position: absolute;
left: 0px;
padding-bottom: 0px;
background: #006400;
float: left;
height: 0;
overflow: hidden;
}
#content #nav li .a {
text-decoration: none;
color: #00CD00;
line-height: 32px;
display: block;
border-right: 1px solid #009800;
}
#nav div a {
text-decoration: none;
color: #00CD00;
line-height: 26px;
display: block;
}
#nav div a:hover {
background: #005400;
}
</style>
</head>
<body>
<div id="content">
<div id="nav">
<ul id="supnav">
<li><a href="#" class="a">好好学习</a>
<div>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
</div>
</li>
</ul>
</div>
</div>
<script type="text/javascript">
var supnav = document.getElementById("supnav");
var nav = document.getElementById("nav");
var btns = document.getElementsByTagName("li");
var subnavs = nav.getElementsByTagName("div");
var paddingbottom = 20;
var defaultHeight = 0;
function drop(obj, ivalue) {
var a = obj.offsetHeight;
var speed = (ivalue - obj.offsetHeight) / 8;
a += Math.floor(speed);
obj.style.height = a + "px";
}
window.onload = function() {
for ( var i = 0; i < btns.length; i++) {
btns[i].index = i;
btns[i].onmouseover = function() {
var osubnav = subnavs[this.index];
var sublinks = osubnav.getElementsByTagName("a");
if (osubnav.firstChild.tagName == undefined) {
var itarheight = parseInt(osubnav.childNodes[1].offsetHeight)
* sublinks.length + paddingbottom;
} else {
var itarheight = parseInt(osubnav.firstChild.offsetHeight)
* sublinks.length + paddingbottom;
}
clearInterval(this.itimer);
this.itimer = setInterval(function() {
drop(osubnav, itarheight);
}, 30);
}
btns[i].onmouseout = function() {
var osubnav = subnavs[this.index];
clearInterval(this.itimer);
this.itimer = setInterval(function() {
drop(osubnav, defaultHeight);
}, 30);
}
}
}
</script>
</body>
</html>