Improving site performance by shipping more CSS

GitHub’s Primer Design System migrated from a CSS-in-JS approach based on styled-components to CSS Modules after growing component counts created client initialization, server-side rendering, and style-update performance problems. The team used an incremental, feature-flagged process that paired CSS Module translations with visual regression testing and staged production rollouts, completing Primer’s component migration by December 2024 and reducing server-side rendering time by 55% and component initialization time by 25%. Extending the change across GitHub required replacing thousands of dynamic `sx` styling props and transitioning imports from a compatibility wrapper to the CSS Module-based Primer package, aided by internal editor tools, codemods, and later Copilot coding agents. Engineers also separated GitHub’s multi-theme support from styled-components through further staged migrations. By June 2026, GitHub had removed `sx`, styled-components, and styled-system from its main product, turning an initial CSS conversion into a broader re-platforming of UI styling and theming while maintaining production stability.

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