<style type="text/css">
.shell{
position:relative;
color:#fff;
height:150px;
width:55px;
display:block
border:1px solid #aaa;
filter:alpha(opacity=50);
margin-right:10px;
}
</style>
<div class="shell">
<div id="up" style="width: 55px; height: 10px; background: red; position: absolute; top: 57px; left: 0;"></div>
<div id="down" style="z-index:1;margin-top: 25px;width:55px;height:80px;overflow:hidden;left: 0;">
<a id="a" href="javascript:" onclick="a(this)">小区垃圾</a>
<a id="b" href="javascript:" onclick="b(this)">违章建筑</a>
<a id="c" href="javascript:" onclick="c(this)">违法摆摊</a>
<a id="d" href="javascript:" onclick="d(this)">噪音污染</a>
<a id="e" href="javascript:" onclick="ee(this)">物业纠纷</a>
</div>
</div>
主要看 position：absolute;、position:relative;和z-index:1; 写的位置
