The extra small option available in Bootstrap 3 was removed in Bootstrap 4 version onwards. Tips You don't need to remember all CSS classes. They can help draw the visitor’s eye, provide them with important information, or persuade them to take action on your site. Navbar Header.navbar-header is the second one main components of the navbar. To work with radio buttons using Bootstrap, we need to set up and configure Bootstrap 4 UI library in our project. As a result, the elements can sit next to each other.