第一种：
<!doctype html>
<html>
<head>
<meta content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" name="viewport">
<meta charset="utf-8">
<title></title>
<style type="text/css">
*{
margin: 0;padding: 0;}
div{
width: 100px;height: 100px;background: black;
position: absolute;left: 50%;top: 50%;
transform:translate(-50%,-50%);
}
</style>
</head>
<body>
<div></div>
</body>
</html>
第二种：
<!doctype html>
<html>
<head>
<meta content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" name="viewport">
<meta charset="utf-8">
<title></title>
<style type="text/css">
*{
margin: 0;padding: 0;}
div{
width: 100px;height: 100px;background: black;
position: absolute;left: 50%;top: 50%;margin: -50px 0 0 -50px;
}
</style>
</head>
<body>
<div></div>
</body>
</html>
第三种：
<!doctype html>
<html>
<head>
<meta content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" name="viewport">
<meta charset="utf-8">
<title></title>
<style type="text/css">
*{
margin: 0;padding: 0;}
body,html{
height: 100%;}
div{
width: 100px;height: 100px;background: black;
position: fixed;
left: 50%;top: 50%;
margin-top: -50px;
margin-left: -50px;
}
</style>
</head>
<body>
<div></div>
</body>
</html>
注意一下，为什么是margin呢？
因为padding是自身的扩大,margin是距离.left top bottom right