# Hydration failed because the initial UI does not match what was rendered on the server. Warning: Expected server HTML to contain a matching <div> in <div>.

- **ID:** `react/hydration-error-undefined`
- **Domain:** react
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Server-rendered HTML structure differs from client's first render, often due to browser-specific APIs (e.g., localStorage, window) causing different output on client vs server.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| react-dom@18.2.0 | active | — | — |
| next@14.0.0 | active | — | — |
| react@18.2.0 | active | — | — |

## Workarounds

1. **Use dynamic import with { ssr: false } in Next.js for components that rely on browser APIs: `const MyComponent = dynamic(() => import('./MyComponent'), { ssr: false })`** (90% success)
   ```
   Use dynamic import with { ssr: false } in Next.js for components that rely on browser APIs: `const MyComponent = dynamic(() => import('./MyComponent'), { ssr: false })`
   ```
2. **Ensure client and server render the same markup by checking for browser environment before rendering: `if (typeof window === 'undefined') return null;` or using `useEffect` to set state after mount with a loading state.** (85% success)
   ```
   Ensure client and server render the same markup by checking for browser environment before rendering: `if (typeof window === 'undefined') return null;` or using `useEffect` to set state after mount with a loading state.
   ```

## Dead Ends

- **Adding suppressHydrationWarning prop to root element** — This only suppresses the warning, doesn't fix the underlying mismatch. The UI may still flash or break. (60% fail)
- **Using useEffect to set state after mount without conditional rendering** — Doesn't prevent the initial mismatch during hydration; only fixes subsequent updates. (50% fail)
- **Disabling SSR entirely for the component** — Overly aggressive; loses SSR benefits and may cause layout shift. (40% fail)