18F currently recommends using the U.S. Web Design System as it is specifically designed to help build fast, accessible, mobile-friendly federal government websites.
Sometimes, projects utilize other CSS frameworks such as:
These frameworks were chosen because they’re relatively unopinionated about design decisions while still providing the helpers that make frameworks essential to fast and accurate frontend work, for example, solutions for responsive design, grids, and common design patterns. In addition, both frameworks, through modular design and excellent documentation, make it easy for the designer or developer to only use the parts that they need, rather than including a hefty library.
Do not use Bootstrap
18F specifically does not recommend using Bootstrap for production work because it can be difficult to adapt its opinionated styles to bespoke design work.