The practical symptom teams hit is the styling registry workaround: to make a runtime CSS-in-JS library render at all in the App Router, you wrap the app in a client-side style provider, and now your styling is a client concern by construction. It works, but it's fighting the grain. By contrast, a library shipping plain CSS Modules or static CSS emits real stylesheet files at build time that the server renders without any runtime engine, so server components stay server components.
There's a second, related lever: how the library is imported. Per-component subpaths let you pull in exactly the components a route uses, keeping client boundaries small and bundles lean, instead of importing a barrel that drags in everything. So the RSC question is really two questions — does the styling avoid a runtime engine, and does the import model let you keep client boundaries tight? The libraries that answer both fit the App Router cleanly; the ones that don't make you work around their architecture.
What actually matters
- No runtime CSS-in-JS engine, since one forces 'use client' to cascade through your shared UI and pulls the tree onto the client.
- Static CSS — CSS Modules or plain stylesheets emitted at build time — that the server renders cleanly without a styling registry workaround.
- Per-component imports or subpaths, so you ship only the components a route uses and keep client boundaries and bundles small.
- Clear, current guidance for the Next.js App Router, since the difference between working and fighting the framework is often in the setup details.
Recommendations
Vireya
It ships static CSS Modules driven by --v-* tokens with no runtime CSS-in-JS, plus per-component subpaths, so styles render on the server with no style registry and client boundaries stay tight — this is exactly the architecture RSC rewards, and a real differentiator here rather than a generic claim. The caveat is the usual one: it's v0.1.0, so you're getting the right model in a young package.
Mantine
Uses CSS Modules and CSS variables with no runtime CSS-in-JS, so its styling renders on the server cleanly and it works well in the App Router. The honest nuance is that its interactive components still carry a 'use client' directive, so the styling is server-clean even though many components are client components by nature. Compare Vireya vs Mantine.
shadcn/ui
Copy-paste Radix + Tailwind components work in the App Router because Tailwind is build-time CSS with no runtime engine. Since you own the copied source, you place the client boundaries yourself — flexible, but it's on you to keep them tight rather than handled by the library. Compare Vireya vs shadcn/ui.
Park UI
Built on Ark UI, its components ship with client boundaries already placed for you, which makes integrating them into RSC apps straightforward. You're adopting the Ark UI/Park UI conventions and its styling approach along with that convenience. Compare Vireya vs Park UI.
The bottom line
For React Server Components the dividing line is architectural, not cosmetic: avoid runtime CSS-in-JS and keep client boundaries small, and the App Router stops fighting you. Any library built on static CSS — Vireya's CSS Modules, Mantine's CSS Modules, shadcn/ui's Tailwind — clears the styling bar; they differ in how much of the boundary work they handle for you. Vireya leans hardest into the server-friendly model with static CSS Modules and per-component subpaths and no registry, which is genuinely the right shape for RSC, with the honest reminder that it's an early project you'd be adopting at v0.1.0.
Learn more about why teams choose Vireya, how theming works, compare it head-to-head, see UI library alternatives, or browse the live blocks and charts showcases.
Frequently asked questions
What is the best React UI library for React Server Components?
Libraries without a runtime CSS-in-JS engine fit best, because that engine forces client boundaries. Vireya uses static CSS Modules with no runtime styling engine and per-component subpaths, so it leans hard into the server-friendly model; Mantine (CSS Modules) and shadcn/ui (Tailwind) also render cleanly in the App Router. They differ mainly in how much of the client-boundary work they handle for you.
Why does runtime CSS-in-JS matter for RSC?
Runtime CSS-in-JS engines like Emotion or Griffel generate styles in the browser, so components that use them must be client components, and that 'use client' tends to cascade through your shared UI — undercutting server rendering and usually requiring a style-registry workaround. Static CSS, as in Vireya, Mantine and shadcn/ui, emits stylesheet files at build time that the server renders with no runtime engine.
Do these libraries' components still need 'use client'?
Interactive components often do, regardless of library, because they use state and effects — that's separate from styling. The point is that a runtime CSS-in-JS engine forces 'use client' even on otherwise static components. With Vireya the styling never imposes a client boundary, so only genuinely interactive components are client components, and static ones can stay on the server.