要对Bootstrap卡设置否定操作，请使用bg-warning类和该卡类，如以下示例所示-
<div class="card bg-warning"> <div class="card-body">Over 20 students failed the final-year exam.</div> </div>
让我们看一个示例，以对Bootstrap 4中的卡设置负面操作-
<!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"> <h3>Result</h3> <div class="card bg-warning"> <div class="card-body">Over 20 students failed the final-year exam.</div> </div> </div> </body> </html>