React Performance: Rendering — Hydration No Flicker. Keywords: hydration mismatch flicker. Platform: React/Next.js. Description: Use inline script to set client-only data before hydration. Do: Inject sync script for client-only values. Don't: Use useEffect causing flash. Good Example: <script dangerouslySetInnerHTML={{ __html: 'el.className = localStorage.theme' }} />. Bad Example: useEffect(() => setTheme(localStorage.theme), []) // flickers. Severity: Medium.