Hydration mismatches are a silent SEO killer.

Hydration mismatches are a silent SEO killer.

When React (or any SSR framework) sends a server-rendered HTML string to the browser, then hydrates it with JavaScript, you get two versions of the DOM existing briefly — one the crawler may cache, one the user ultimately sees. If they don't match perfectly, React throws a hydration warning. Most developers dismiss these warnings. SEOs should not.

Hydration mismatches often happen when:
- Components read from `window` or `document` during SSR (which don't exist server-side)
- Date/time formatting differs between server timezone and client timezone
- User-specific data (auth state, locale) is injected too early

Googlebot uses a relatively old version of Chromium. It may cache the pre-hydration HTML before JavaScript finishes executing. If your pre-hydration HTML is incomplete — missing product prices, empty category descriptions, placeholder meta tags — that's what ends up in the index.

Test this by fetching your page with `curl` and comparing the raw HTML against a full JS-rendered screenshot. Any content gap is a risk.