Bootstrap 4 Button .btn-outline-warning类
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-
示例
<!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>