Bootstrap UI

Bootstrap is a CSS framework for building responsive web interfaces. Mark Otto and Jacob Thornton created it at Twitter in 2011 to standardize internal front-end tooling, then open-sourced it on GitHub where it became one of the most widely adopted front-end frameworks. Bootstrap provides a mobile-first grid system, prebuilt UI components (navigation, forms, modals, cards), and utility classes for spacing, typography, and layout.

The current release is Bootstrap 5.3, which dropped the jQuery dependency, adopted CSS custom properties for runtime theming (including dark mode), and uses vanilla JavaScript for interactive components like dropdowns and modals. The framework is built on Sass, with variables and mixins that allow customization of colors, breakpoints, spacing scales, and component styles before compilation.

The official documentation includes examples, component demos, and migration guides. The source code is on GitHub under the MIT license.