Bootstrap

Developed by Jacob Thornton and Mark Otto at Twitter as a framework to promote consistency across internal tools, Bootstrap is an open-source framework containing CSS and JavaScript-based templates for interface components. 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).