要将元素设置为具有圆角，请使用Bootstrap 4中的rounded-bottom类。
设置圆形底部类-
<div class="demo rounded-bottom"></div>在上面的<div>中，我还设置了另一个类来设置元素的样式-
.demo {
width: 100px;
height: 120px;
margin: 10px;
background-color: orange;
}您可以尝试运行以下代码来实现rounded-bottom类-
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://cdn.staticfile.org/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<style>
.demo {
width: 100px;
height: 120px;
margin: 10px;
background-color: orange;
}
</style>
</head>
<body>
<div class="container">
<h2>Rectangles</h2>
<p>We have two rectangles with rounded corners (bottom):</p>
<div class="demo rounded-bottom"></div>
<div class="demo rounded-bottom"></div>
</div>
</body>
</html>