css: 边宽弧度

<div class="box">
<div class="home_top"> 这里是内容 </div>
</div>
.box {  250px; margin: auto; overflow: hidden; }
.home_top { position: relative;  100%; height: 100px; text-align: center; line-height: 50px; }
.home_top:after {  140%; height: 100px; position: absolute; left: -20%; top: 0; z-index: -1; content: ''; border-radius: 0 0 50% 50%; background: linear-gradient(#2CBAF9, #04A9F4); }

效果图1：底部凸圆弧

.focus{ position: relative;}
.focus:after{ position:absolute; left:0; right:0; bottom:-11px; content:""; z-index:1; height:20px; 100%; border-radius:50%; background:#fff;}

效果图2：底部凹圆弧