Similar to its predecessor, Bootstrap 4 allows you to create mobile-friendly websites without too much additional development work. By default, Bootstrap is designed to work across all resolutions and screen sizes, from mobile to tablet to a desktop. In fact, Bootstrap's mobile-first design philosophy implies that its components must display and function correctly at the smallest screen size possible. The reasoning behind this is simple. Think about developing a website without consideration for small mobile screens. In this case, you are likely to pack your website full of buttons, labels, and tables. You will probably only discover any usability issues when a user attempts to visit your website using a mobile device, only to find a small web page that is crowded with buttons and forms. At this stage, you will be required to rework the entire user interface to allow it to render on smaller screens. For precisely this reason, Bootstrap promotes a bottom-up approach, forcing developers to get the user interface to render correctly on the smallest possible screen size, before expanding upward.