设置一个长是宽的两倍的长方形，然后根据需要利用border-radius去除两个顶角。
实例（画了四个半圆）：
代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
div { width: 100px; height: 50px; }
.div1 { border-radius: 100px 100px 0px 0px; background-color: red; }
.div2 { border-radius: 0px 0px 100px 100px; background-color: blue; }
.div3, .div4 { width: 50px; height: 100px; display: inline-block; }
.div3 { border-radius: 100px 0px 0px 100px; background-color: gold; }
.div4 { border-radius: 0px 100px 100px 0px; background-color: green; }
</style>
</head>
<body>
<div class="div1"></div>
<div class="div2"></div>
<div class="div3"></div><div class="div4"></div>
</body>
</html>
