第一种：将固定大小的div框相对窗口水平垂直居中，改变浏览器窗口大小时，依然保持水平垂直居中；
<!doctype html>
<html lang="en">
<head>
<title>水平垂直居中</title>
<meta charset="utf-8">
<style type="text/css">
.out{width: 0px;
height: 0px;
position: absolute;
left: 50%;
top: 50%;
}
.in{height: 300px;
width: 300px;
position: absolute;
left: -150px;
top: -150px;
background-color: #999;
}
</style>
</head>
<body>
<div class="out">
<div class="in"></div>
</div>
</body>
</html>
第二种：让一个自适应大小的div相对浏览器窗口水平垂直居中
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>自适应剧中</title>
<style type="text/css">
.middle{
height: 300px;
width: 300px;
border: 1px solid;
display: table-cell;
text-align: center;
vertical-align: middle;
}
.in{
background-color: red;
height: auto;
width: auto;
min-width: 0px;
min-height: 0px;
display: inline;
}
</style>
</head>
<body>
<div class="middle">
<div class="in">内容自适应水平垂直居中</div>
</div>
</body>
</html>
