Bootstrap .btn-primary类
.btn-primary类提供了额外的视觉效果，并在一组按钮中标识了主要操作：
示例
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Example</title>
<meta name = "viewport" content = "width=device-width, initial-scale=1">
<link rel = "stylesheet" href = "https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css">
<script src = "https://cdn.staticfile.org/jquery/3.3.1/jquery.min.js"></script>
<script src = "https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script>
</head>
<body>
<button type = "button" class = "btn btn-primary">Primary Button</button>
</body>
</html>