How We Migrated 150+ Components from MUI to Carbon Design System v11 (And Reduced Accessibility Violations by 78%)

A practical account of a large-scale UI library migration — what we planned, what broke, and what we'd do differently. When our team decided to migrate from Material UI to Carbon Design System v11, the scope was clear: 150+ components across an enterprise DNS platform, hundreds of thousands of lines of React code, and zero tolerance for regressions on production traffic. What wasn't clear was how much the migration would teach us about the hidden cost of design system debt. Here's what we learned. Why We Left MUI MUI is excellent. We're not here to trash it. But for a product shipping inside IBM's ecosystem, we were maintaining two parallel design languages — MUI's Material-influenced components and IBM's Carbon — in ways that created real friction: Accessibility inconsistencies : MUI and Carbon have different ARIA patterns. Running them side-by-side meant our a11y audit results were impossible to reason about systematically. Token misalignment : MUI's theming system uses its own token