Bootstrap is known for popularizing the focus on responsive design among web developers. It promoted the now-ubiquitous concept of mobile-first and provided the right tools for its easy implementation. It did so by introducing a grid – partitioning the screen into columns (invisible to the end user’s eye). With Bootstrap, developers don’t have to build separate projects around adjusting a site for smaller screen sizes. They just need to incorporate the necessary Bootstrap classes, and the design adjusts itself.