带有Boot-Strap 4类危险等级的Bootstrap卡
要给Bootstrap卡着色，我们使用上下文类。
对于危险动作，将bg-danger上下文类别与卡片类别一起使用-
<div class="card bg-danger"> <div class="card-body">Danger! High Voltage!</div> </div>
您可以尝试运行以下代码来实现card-danger类-
示例
<!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>
</head>
<body>
<div class="container">
<h2>Messages</h2>
<div class="card">
<div class="card-body">How you doing?</div>
</div><br>
<div class="card bg-danger">
<div class="card-body">Danger! High Voltage!</div>
</div><br>
</div>
</body>
</html>