Primer is GitHub's design system, grown from an internal CSS framework into a multi-platform system covering React, Rails ViewComponents, a Figma library, and a standalone CSS toolkit, plus the Octicons icon set.
It's strongest for building genuinely developer-facing product UI, components like Action List and Action Menu ship with full keyboard navigation and ARIA correctness built in, patterns proven at GitHub's own scale rather than theoretical accessibility guidance.
Developers building a developer-tool or code-adjacent product who want components proven at GitHub's scale, installable directly via primer/react or primer/css.
It spans multiple implementations (React, Rails, CSS-only), worth checking which matches the project's actual stack before adopting.
The design system that powers GitHub.
