为什么我在li加了border ,只有前四个添加了，后四个没有 怎么弄
<style>
#main{
width:312px;
height:519px;
margin-left:auto;
margin-right:auto;
background-color:#C06;
}
#main ul{
margin:0px;
padding:0px;
}
#main li{
list-style:none;
border-bottom:3px #3300FF dashed;
line-height:36px;
height:36px;
background-color:#0FF;
}
.up{
float:left;
width:312px;
border-bottom:none;
}
.down{
float:right;
width:312px;
border-bottom:none;
}
</style>
</head>//这是body
<body>
<div id="main">
<ul>
<li>
<div class="up">ss</div>
<div class="down">da</div>
</li>
<li>
<div class="up">ss</div>
<div class="down">da</div>
</li>
<li>
<div class="up">ss</div>
<div class="down">da</div>
</li>
<li>
<div class="up">ss</div>
<div class="down">da</div>
</li>
<li>
<div class="up">ss</div>
<div class="down">da</div>
</li>
</ul>
</div>
</body>
我想做的效果
