<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
#box{
margin: 0 auto;
width: 150px;
height: 150px;
border: 1px solid #FF0000;
position: relative;
}
#box #text{
width: 50px;
height: 50px;
position: absolute;
left: 0;
right: 0;
top: 0;
bottom: 0;
margin: auto;
}
</style>
</head>
<body>
<div id="box">
<div id="text">
123
</div>
</div>
</body>
</html>
left:0; right:0; top:0; bottom:0; margin：auto;会使text居中，如果text没有宽高，则会继承父集的宽高。适合body 内的遮罩，
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