Bootstrap
Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains HTML, CSS, and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components, enabling developers to quickly build modern, consistent websites and web applications. Originally created by Twitter, it is now maintained by a community of developers and is one of the most popular front-end frameworks globally.
Developers should learn Bootstrap when they need to rapidly prototype or build responsive web interfaces with minimal custom CSS, as it provides pre-styled components and a grid system that works across devices. It is particularly useful for projects with tight deadlines, teams requiring consistency, or developers new to front-end design who want a solid foundation without deep CSS expertise. Common use cases include building admin dashboards, marketing websites, and internal tools where speed and responsiveness are priorities.