让一个已知宽高的div元素水平居中
<style>
#div1{
200px;
height:200px;
background:#F00;
margin:0 auto;
}
</style>
</head>
<body>
<div id="div1">
</div>
</body>
</html>
让一个已知宽高的div元素垂直居中
<style>
#div1{
300px;
height:200px;
background:#F00;
position:absolute;
top:50%;
margin-top:-100px;
}
</style>
</head>
<body>
<div id="div1">
</div>
</body>
</html>