dom 左右两侧得广告（兼容IE FF）

<!doctype html>

<html>

<head>

<meta charset="utf-8">

<title>无标题文档</title>

<style>

body{

    padding:0px;

    margin:0px;

    }

#wrap{

    height:2000px;

    }

.ad{

    height:200px;

    width:50px;

    background:pink;

    position:fixed;

    }

.left{

    left:0px;

    }

.right{

    right:0px;

    }

</style>

<script type="text/javascript">

window.onload = function ()

{

    var oad = document.getElementsByClassName('ad');

    oad[0].style.top = oad[1].style.top = (document.documentElement.clientHeight - oad[0].offsetHeight)/2 + 'px';

};

</script>

</head>

<body>

    <div id="wrap">

        <div class="ad left"></div>

        <div class="ad right"></div>

    </div>

</body>

</html>