jquery 保持头部固定

<!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>导航固定</title>
<script src="/ajaxjs/jquery1.3.2.js"language="javascript" type="text/javascript"></script>
<script>
$(function(){    
  $(window).scroll(function() {
    //$("body").css({"background-position":"center "+$(window).scrollTop()+""});
    if($(window).scrollTop()>=250){
        $(".nav").addClass("fixedNav");
    }else{
        $(".nav").removeClass("fixedNav");
    } 
  });
});
</script>
<style>
*{
    margin:0px;
    padding:0px;
}
div.nav{
    background:#000000;
    height:57px;
    line-height:57px;
    color:#ffffff;
    text-align:center;
    font-family:"微软雅黑", "黑体";
    font-size:30px;
}

div.fixedNav{
    position:fixed;
    top:0px;
    left:0px;
    width:100%;
    z-index:100000;
    _position:absolute;
    _top:expression(eval(document.documentElement.scrollTop));
}
</style>
</head>
<body>   
<div class="header" style="background:#CCCC00;height:250px;"></div>
<div class="nav">
<p>导航固定</p>
</div>
<div class="content" style="background:#0099FF; height:2000px;"></div>
</body>
</html>


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