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;
}
