1.transform :translate(-50%,-50%)
<!DOCTYPE html>
<html lang="en"> <head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
body,
html {
width: 100%;
height: 100%;
margin: 0;
padding: 0
} .box {
width: 100%;
height: 100%;
background: red
} .div1 {
width: 200px;
height: 200px;
background: black;
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%, -50%)
}
</style>
</head> <body>
<div class="box">
<div class="div1"></div>
</div> </body> </html>
2.top，bottom,left,right和margin：auto
<!DOCTYPE html>
<html lang="en"> <head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
body,
html {
width: 100%;
height: 100%;
margin: 0;
padding: 0
} .box {
width: 100%;
height: 40%;
background: red;
position: relative
} .div1 {
width: 200px;
height: 200px;
background: black;
position: absolute;
left: 0;
right: 0;
top: 0;
bottom: 0;
margin: auto
}
</style>
</head> <body>
<div class="box">
<div class="div1"></div>
</div> </body> </html>