动态效果如图所示： 
第一种实现方法：
<!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>JavaScript操作CSS:Style</title>
<style type="text/css">
li{
font-size: 12px;
color: #ffffff;
background-image: url(images/bg1.gif);
background-repeat: no-repeat;
height: 33px;
104px;
text-align: center;
line-height:38px;
list-style:none;
float:left;
}
</style>
<script type="text/javascript">
function changeBg1(currObj){
//style="background-image:url(images/bg2.gif)"
//CSS style属性 background-image属性
currObj.style.backgroundImage="url(images/bg2.gif)";
//JavaScript style对象 backgroundImage对象
currObj.style.color="yellow";
currObj.style.fontSize="20px"
}
function changeBg2(currObj){
currObj.style.backgroundImage="url(images/bg1.gif)";
currObj.style.color="#ffffff";
currObj.style.fontSize="12px"
}
</script>
</head>
<body>
<ul>
<li onmouseover="changeBg1(this)" onmouseout="changeBg2(this)" style="background-image:url(images/bg2.gif)">资讯动态</li>
<li onmouseover="changeBg1(this)" onmouseout="changeBg2(this)">产品世界</li>
<li onmouseover="changeBg1(this)" onmouseout="changeBg2(this)">市场营销</li>
</ul>
</body>
</html>
第二种实现方法：
<!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>JavaScript操作CSS:Style</title>
<style type="text/css">
li{
font-size: 12px;
color: #ffffff;
background-image: url(images/bg1.gif);
background-repeat: no-repeat;
height: 33px;
104px;
text-align: center;
line-height:38px;
list-style:none;
float:left;
}
</style>
<script type="text/javascript">
//匿名函数
//页面加载完调用
window.onload = function(){
var liArr = document.getElementsByTagName("li");
for(var i=0; i<liArr.length; i++){
//动态绑定事件
liArr.item(i).onmouseover = function(){
this.style.backgroundImage = "url(images/bg2.gif)";
this.style.color = "yellow";
this.style.fontSize = "20px";
}
liArr.item(i).onmouseout = function(){
this.style.backgroundImage = "url(images/bg1.gif)";
this.style.color = "#ffffff";
this.style.fontSize = "12px";
}
}
}
</script>
</head>
<body>
<ul>
<li >资讯动态</li>
<li>产品世界</li>
<li>市场营销</li>
</ul>
</body>
</html>
第三种实现方法：（推荐）
<!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>JavaScript操作CSS:Style</title>
<style type="text/css">
li{
font-size: 12px;
color: #ffffff;
background-image: url(images/bg1.gif);
background-repeat: no-repeat;
height: 33px;
104px;
text-align: center;
line-height:38px;
list-style:none;
float:left;
}
.over{
background-image:url(images/bg2.gif);
color:yellow;
font-size:20px;
}
.out{
background-image:url(images/bg1.gif);
color:#ffffff;
font-size:12px;
}
</style>
<script type="text/javascript">
//通过js操作css
window.onload = function(){
var liArr = document.getElementsByTagName("li");
for(var i=0; i<liArr.length; i++){
//动态绑定事件
liArr.item(i).onmouseover = function(){
this.className = "over";
}
liArr.item(i).onmouseout = function(){
this.className = "out";
}
}
}
</script>
</head>
<body>
<ul>
<li class="">资讯动态</li>
<li>产品世界</li>
<li>市场营销</li>
</ul>
</body>
</html>