Bootstrap

Bootstrap is a free, open-source CSS framework for building responsive, mobile-first websites quickly. It provides a grid system for layouts and ready-made styles and components such as buttons, forms, navigation bars, cards, modals and alerts.

Developers apply Bootstrap by adding its CSS and JavaScript files and using its class names in HTML, for example class="btn btn-primary". It speeds up development and gives a consistent look across browsers, and its design can be customised with Sass variables.