這里有top和margin-top的區別，top(left,right,bottom)是絕對定位，要用position,margin-top是相對定位，相對於相鄰的元素或者父元素。
代碼如下:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" >
<title>square at center</title>
<style type="text/css">
#square{
position:absolute;
top:50%;
left:50%;
margin-top:-50px;
margin-left:-50px;
width:100px;
height:100px;
background-color:red;
}
</style>
</head>
<body>
<div id="square">
</div>
</body>
</html>