效果：
代码：
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>ccc</title>
<style type="text/css">
.box {
position: relative;
width: 100%;
height: 200px;
text-align: center;
color: #fff;
background-color: yellow;
}
.box:after {
position: absolute;
left: 0;
right: 0;
bottom: -100px;
z-index: -1;
content: ' ';
height: 200px;
width: 100%;
border-radius: 0 0 50% 50%;
background-color: yellow;
}
</style>
<div class='box'>hello world</div>
</body>
</html>