How to write couplet advertising code using css

<!-- css 代码-->
.couplet_ad
/* 底部固定*/
{position:fixed;bottom:auto; top:0; width: 120px; height: 230px; z-index:99999; margin-top:158px;}
html .couplet_ad
/* IE6 底部固定*/
{_position:absolute;
_bottom:auto;
_top:expression(eval(document.documentElement.scrollTop));}
.couplet_ad a{ display:block; cursor:pointer;}
#ad_left{ left: 0px;}
#ad_right{ right: 0px;}
 <!-- html代码 -->
 <!-- 对联广告左边 -->
 <div class="couplet_ad" id="ad_left">
<a target="_blank" href=""><img src="dl.jpg"></a>
<a onClick="ad_left();">关闭</a>
</div>
<!-- 对联广告右边 -->
<div class="couplet_ad" id="ad_right">
<a target="_blank" href=""><img src="dl.jpg"></a>
<a  onClick="ad_right();">关闭</a>
</div>
<!-- js关闭按钮代码 -->
<script language="javascript">
function ad_left(){
document.getElementById(&#39;ad_left&#39;).style.display="none";
}
function ad_right(){
document.getElementById(&#39;ad_right&#39;).style.display="none";
}
</script>