要在指示警告的按钮上设置轮廓，您需要使用Bootstrap中的btn-outline类。
对于我的类，我选择了以下按钮-
<button type="button" class="btn btn-outline-warning"> Warning (Errors) </button>
上面的按钮具有btn-outline-warning类，该类使我们可以添加橙色轮廓来说明警告措施-
您可以尝试运行以下代码来实现btn-outline-warning类-
<!DOCTYPE html> <html> <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> <h2>Warning</h2> <p>The following are the warning messages:</p> <ul class = "list-group"> <li class = "list-group-item">semi-colon (;) missing</li> <li class = "list-group-item">infinite loop</li> </ul> <p>For more warning messages:</p> <button type="button" class="btn btn-outline-warning">Warning (Errors)</button> </body> </html>