css中position:fixed实现div在窗口上下左右居中

div{
position:fixed;
margin:auto;
left:0;
right:0;
:0;
bottom:0;
width:200px;
height:150px;
}

<style type="text/css">
.dialog{
position: fixed;
_position:absolute;
z-index:1;
: 50%;
left: 50%;
margin: -141px 0 0 -201px;
width: 400px;
height:280px;
border:1px solid #CCC;
line-height: 280px;
text-align:center;
font-size: 14px;
background-color:#F4F4F4;
overflow:hidden;
}
</style>
<div class="dialog">我是在窗口正中央的，呵呵！</div>

.dialog{
position: fixed;
_position:absolute; / hack for IE6 /
z-index:1;
: 50%;
left: 50%;
margin: -141px 0 0 -201px;
width: 400px;
height:280px;
border:1px solid #CCC;
line-height: 280px;
text-align:center;
font-size: 14px;
background-color:#F4F4F4;
overflow:hidden;
}