利用border-radius和border完成：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>test</title>
<style>
.semicircle {
width: 100px;
height: 50px;
border-radius: 0 0 50px 50px;
background: #111111;
}
.semicircle2 {
width: 100px;
height: 50px;
border-radius: 0 0 50px 50px;
box-sizing: border-box;
border: 2px solid #111111;
border-top: none;
}
</style>
</head>
<body>
<div class="semicircle">
</div>
<br><br>
<div class="semicircle2">
</div>
<br><br>
</body>
</html>
