GitHub styleguide.
Primer is GitHub’s internal CSS framework. It includes basic global styling for typography, small components like buttons and tabs, and our general guidelines for writing HTML and CSS. It’s been used internally at GitHub for years now.
Open-sourcing Primer means sharing and learning with the community. There’s a lot we can improve upon in Primer, and help is always appreciated. While we don’t currently plan on building this out as a competitor to other front-end frameworks, we will occasionally add, remove, or modify things.