bootstrap

Bootstrap is a front-end framework for building responsive, mobile-first websites and web apps. The README describes it as a sleek, intuitive, and powerful framework for faster and easier web development. Concretely, it gives you a ready-made set of HTML, CSS, and JavaScript building blocks — buttons, forms, navigation bars, grids, modals, and so on — that look polished out of the box and adapt to different screen sizes from phones up to desktops.

It works by shipping compiled CSS and JavaScript files you drop into your web project, plus the original Sass/SCSS source if you want to customize colors, spacing, breakpoints, and so on through configuration variables. The downloaded package includes a base bootstrap.css plus narrower bundles like a grid-only build, a CSS reset called bootstrap-reboot, and utility classes — each available in normal, minified, and right-to-left variants, with source maps for browser dev tools. The bundled JavaScript files include Popper for positioning floating elements like tooltips and dropdowns, and there's also an ESM build for modern bundlers.

You'd use Bootstrap when you want a usable, consistent UI across devices without writing all the CSS yourself, or when you need a baseline you can extend with your own design tokens through Sass. Installation options are flexible: download the latest release, clone the repo, or pull it via a package manager such as npm, yarn, Bun, Composer, or NuGet. Documentation is built with Astro and hosted on GitHub Pages at getbootstrap.com, with search powered by Algolia DocSearch. The current default branch develops Bootstrap 5; Bootstrap 4 lives on a separate v4-dev branch. The repo language is MDX, which is what the docs are written in.