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, as well as optional JavaScript plugins. It helps developers quickly design and customize responsive sites with a consistent look and feel.
Developers should learn Bootstrap when building responsive web applications that need to work across various devices and screen sizes, especially for rapid prototyping or when consistency in UI components is crucial. It's widely used in projects where time-to-market is important, such as startups, internal tools, or content-heavy websites, as it reduces the need for custom CSS and ensures cross-browser compatibility.