Improving site performance by shipping more CSS

GitHub's Primer Design System migrated from CSS-in-JS to CSS Modules to address performance issues caused by the explosion of components on certain pages, which had slowed initial page loads and server-side rendering. CSS Modules allows styles to be authored alongside component code and shipped as static stylesheets, eliminating runtime costs on both client and server. The team implemented an incremental migration strategy using feature flags and visual regression tests to safely update each component without disrupting existing GitHub usage.