GitHub ditches CSS-in-JS and gets 55% faster server-side rendering
Improving site performance by shipping more CSS
GitHub migrated github.com from CSS-in-JS to CSS Modules, cutting server-side rendering time by 55% and component initialization by 25%. The incremental rollout used feature flags and visual regression tests. After migrating Primer components, the team removed 7,760 sx props and eliminated styled-components, finishing in June 2026 without breaking the site.
What looked at first like a CSS migration turned out to be a gradual re-platforming of how GitHub styles, themes, and ships UI at scale.