Copilot · GitHub · GitHub Blog
Improving site performance by shipping more CSS
Compiled by KHAO Editorial — aggregated from 1 source. See llms.txt for citation guidance.
★ Tier-1 Source
The Primer Design System powers many of the experiences you see on GitHub today.
Key facts
- The work kicked off April 2025 with a peak of ~7,760 sx props to be migrated; they wouldn’t see it realized until May 2026
- A rotation of 8 engineers migrated 6,419 props over the course of 6 months, observing Server-Side Rendering time performance gains ranging from 1% up to 22% in some pages
- GitHub has been running on 100% CSS modules as of June 2026
- By December 2024, all components in Primer were migrated over to CSS Modules using this process
Summary
Back in 2023, the number of components on certain pages began to explode. Initial page loads took longer due to styles being initialized on the client. Updates to styles grew out of control as component count grew on a page. It became clear that the Primer team needed to address the issue at the source.