div水平垂直居中方法一：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
#test{
position: absolute;
width: 100px;
height: 100px;
background: pink;
left: 0;
right: 0;
bottom: 0;
top: 0;
margin: auto;
}
</style>
</head>
<body>
<div id="test"></div>
</body>
</html>
div水平垂直居中方法二：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
#test{
position: absolute;
width: 100px;
height: 100px;
background: pink;
left: 50%;
top: 50%;
/*margin-left和margin-top的值为宽度和高度的一半*/
margin-left: -50px;
margin-top: -50px;
}
</style>
</head>
<body>
<div id="test"></div>
</body>
</html>
img水平垂直居中方法：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
html,body{
height: 100%;
}
body{
text-align: center;
}
body:after{
content: "";
display: inline-block;
height: 100%;
vertical-align:middle;
}
img{
vertical-align: middle;
}
</style>
</head>
<body>
<img src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1537119514734&di=5f0585bf2c64ae73b202f2cf98751cd5&imgtype=0&src=http%3A%2F%2Ftgi13.jia.com%2F116%2F258%2F16258758.jpg"/>
</body>
</html>