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>
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