css div 垂直水平居中，并完成 div 高度永远是宽度的一半（宽度可以不指定）
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
<style>
* {
margin: 0;
padding: 0;
}
html,
body {
width: 100%;
height: 100%;
}
.outer {
width: 400px;
height: 100%;
background: blue;
margin: 0 auto;
display: flex;
align-items: center;
}
.inner {
position: relative;
width: 100%;
height: 0;
padding-bottom: 50%;
background: red;
}
.box {
position: absolute;
width: 100%;
height: 100%;
display: flex;
justify-content: center;
align-items: center;
}
</style>
</head>
<body>
<div class="outer">
<div class="inner">
<div class="box">hello</div>
</div>
</div>
</body>
</html>
实现一个大小为父元素宽度一半的正方形
.outer {
width: 400px;
height: 600px;
background: red;
}
.inner {
width: 50%;
padding-bottom: 50%;
background: blue;
}
CSS实现自适应正方形、等宽高比矩形
a.双重嵌套，子绝父相
.outer {
padding-top: 50%;
height: 0;
background: #ccc;
width: 50%;
position: relative;
}
.inner {
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
background: blue;
}
b.padding撑高画正方形
.outer {
width: 400px;
height: 600px;
background: blue;
}
.inner {
width: 100%;
height: 0;
padding-bottom: 100%;
background: red;
}
c.相对于视口VW VH
.inner {
width: 1vw;
height: 1vw;
background: blue;
}
d.伪元素设置margin-top
.inner {
width: 100px;
overflow: hidden;
background: blue;
}
.inner::after {
content: "";
margin-top: 100%;
display: block;
}
实现两栏布局
原文：https://www.cnblogs.com/zgl1224/p/14617829.html