css固定广告栏

<div style="position: fixed; left: 50%; top: 100px; margin-left: -621px;">
    <div>
        <img src="images/duilian.jpg" alt=""/>
    </div>
    <div style="background-color: #cccccc; padding: 10px 0; text-align: center;">
        <a href="#">关闭</a>
    </div>
</div>
<div style="position: fixed; left: 50%; top: 100px; margin-left: 556px;">
    <div>
        <img src="images/duilian.jpg" alt=""/>
    </div>
    <div style="background-color: #cccccc; padding: 10px 0; text-align: center;">
        <a href="#">关闭</a>
    </div>
</div>