CSS弧形背景

.pure_top{
width:100%;
height: 100px;
position: relative;
z-index: -1;
overflow: hidden;
}
.pure_top::after{
content: "";
width:140%;
height:100px;
position: absolute;
left:-20%;
top:0;
z-index: -1;
border-radius: 0 0 50% 50%;
background: red;