.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：
<!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>