GitHub Cut Server Render Time 55% by Ditching CSS-in-JS, With Copilot Agents Finishing the Migration
DanWahlin · x · 2026-09-26
GitHub detailed its full migration of github.com from CSS-in-JS to CSS Modules. Exploding component counts since 2023 caused slower first loads (client-side style init), degraded SSR (style collection shifting to server), and uncontrolled style churn. The Primer team picked CSS Modules for native CSS with colocation and local-by-default class names, eliminating both client and server style costs. Result: 55% faster server render and 25% faster component init, with Copilot coding agents helping finish the last stretch of the migration.
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