方案一（最佳方案）：
<!DOCTYPE HTML>
<html>
<head>
<title>test</title>
<meta charset="utf-8">
<style>
#demo{
width:300px;
height:300px;
position: relative;
background-color: #ff6600;
}
#indemo{
width:100px;
height:100px;
position: absolute;
left:0;right:0;top:0;bottom:0;
margin:auto;
background-color: white;
}
</style>
</head>
<body>
<div id = 'demo'>
<div id = 'indemo'></div>
</div>
</body>
</html>
方案二（该方案必须已知indemo的宽度和高度）：
#demo{
width:300px;
height:300px;
position: relative;
background-color: #ff6600;
}
#indemo{
width:100px;
height:100px;
position: absolute;
left:50%;
top:50%;
margin-left: -50px;
margin-top: -50px;
background-color: white;
}
方案三（不使用定位）：
#demo{
width:300px;
height:300px;
display: table-cell;
vertical-align: middle;
background-color: #ff6600;
}
#indemo{
width:100px;
height:100px;
margin:0 auto;
background-color: white;
}
