Primer CSS
The CSS design system that powers GitHub
...Our approach to CSS is influenced by Object-Oriented CSS principles, functional CSS, and BEM architecture. Styles can be mixed and matched to achieve many different layouts, independent of their location. These styles fall into three categories. Single-purpose, immutable styles, that do one thing well. Abstracted patterns for frequently used visual styles. Primer CSS is built upon systems that form the foundation of our styles such as spacing, typography, and color. This systematic approach helps ensure our styles are consistent and interoperable with each other. ...