Bootstrap

Bootstrap is the absolute, legendary, immortal grandfather of all modern CSS frameworks and component libraries. Originally built by engineers at Twitter in 2011 to unify their internal tools, it completely exploded across the internet. It is mathematically responsible for teaching an entire generation of developers how to build "Mobile-Responsive" websites using its famous 12-column grid system. Its absolute biggest differentiator is "Universal Familiarity." It is the Latin of web development. Every single web developer on earth knows what 'class="btn btn-primary"' means. A massive corporate IT department can hire 50 random developers, and on day one, all 50 of them instantly know how to use Bootstrap to build a functioning admin dashboard without needing a single hour of training. Because it has existed for over a decade, its "Ecosystem and Stability" are terrifyingly massive. There are millions of free Bootstrap themes, templates, and plugins. It is not tied to React; it is fundamentally raw HTML/CSS/JS, meaning it works perfectly whether a developer is building a modern React app, an ancient PHP application, or a simple static HTML brochure website.