起因是因为一个人问我怎么实现水平垂直居中，我回答说margin：0 auto然后再align-item：center，额结局就是他说我说错了，还说我是不是没做过，啊啊啊啊啊啊很不开心，很羞耻，所以马上就百度了一下。后来总结了一下三种。
1.用display中的table-cell
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>div居中</title>
<link rel="stylesheet" type="text/css" href="test.css">
</head>
<style>
.father{
width: 800px;
height: 300px;
border: 1px solid red;
display: table-cell;
vertical-align: middle;
}
.div_con{
width: 100px;
height: 100px;
border: 1px solid red;
background-color: yellow;
margin: 0 auto;
}
</style>
<body>
<div class="father">
<div class="div_con">
</div>
</div>
</body>
</html>
2.用定位
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>div居中</title>
<link rel="stylesheet" type="text/css" href="test.css">
</head>
<style>
.father{
width: 800px;
height: 300px;
border: 1px solid red;
position: relative;
}
.div_con{
width: 100px;
height: 100px;
border: 1px solid red;
position: absolute;
left: 50%;
top:50%;
background-color: yellow;
margin-left: -50px;/*子div宽度的一半*/
margin-top: -50px; /*子div高度的一半*/
}
</style>
<body>
<div class="father">
<div class="div_con">
</div>
</div>
</body>
</html>
3.另外一种定位（最常用）
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>div居中</title>
<link rel="stylesheet" type="text/css" href="test.css">
</head>
<style>
.father{
width: 800px;
height: 300px;
border: 1px solid red;
position: relative;
}
.div_con{
width: 100px;
height: 100px;
border: 1px solid red;
position: absolute;
margin: auto;
top: 0;
right: 0;
left: 0;
bottom: 0;
background-color: yellow;
}
</style>
<body>
<div class="father">
<div class="div_con">
</div>
</div>
</body>
</html>