<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<style>
.partent{
width: 200px;
height: 200px;
border: 1px solid #ccc;
position: relative;
/* 3-1.inline-block + text-align + table-cell + vertical-align（单元格方式） */
/* text-align:center;
display:table-cell;
vertical-align:middle; */
/* 4.flex + justify-content + align-items（弹性模型） */
display: flex;
justify-content: center; /* 水平居中 */
align-items:center; /*垂直居中*/
}
.cjild{
width: 50px;
height: 50px;
border: 1px solid #ccc;
/* 1.定位 + margin */
/* position: absolute;
left: 50%;
top: 50%;
margin:-25px 0 0 -25px */
/* 2.定位 + margin */
/* margin: auto;
position: absolute;
top: 0;
right: 0;
bottom: 0;
left: 0; */
/* 3-2 .inline-block + text-align + table-cell + vertical-align（单元格方式） */
/* display:inline-block; */
}
</style>
<body>
<div class="partent">
<div class="cjild"></div>
</div>
</body>
</html>
