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What causes hydration mismatches, and how do you handle them?

Q: What causes hydration mismatches, and how do you handle them?

A **hydration mismatch** occurs when the HTML React rendered on the **server** differs from what the **client** produces on its first render pass. Common causes: `Date.now()`/`Math.random()`, reading `window`/`localStorage`, locale or timezone differences, or invalid HTML nesting (e.g., a ` ` inside a ` `). server HTML must equal the client's first render — else mismatch server HTML <text x="125" y="82" fill="curr

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