# Hydration Risk Begins When Server and Browser Output Diverge ## Claim Hydration becomes an SEO risk when the browser replaces server HTML with meaningfully different content or layout. <!-- atomicity: 1 idea ✓ --> ## Scope & conditions - This applies to JavaScript frameworks that send server HTML and then attach or replace behavior in the browser. - Hydration itself is usually SEO-neutral because content can arrive in the initial HTML. - Less JavaScript reduces the mismatch surface but doesn't guarantee that server and browser states agree. ## Evidence [[3. Resources/Readwise/Articles/Hydration and SEO How It Works and Why It Matters]] explains that mismatches can worsen INP and CLS or cause Google to index server content users never see after the browser re-renders it. ## Links - [[Zet · Headless WordPress improves Core Web Vitals]] — contrasts: a headless architecture improves performance only when its client rendering doesn't introduce mismatch work. - [[Watching a System Run Beats Reading It]] — applies-in: rendered-versus-raw crawling tests what users and crawlers actually receive instead of trusting source HTML alone. - [[Platform Friction Sets the Real AI Productivity Gain]] — extends: framework rendering choices add delivery behavior that implementation speed doesn't expose. - [[3. Resources/MOCs/Product & Startup Instincts MOC]] — applies-in: the MOC lists this note under Permanent Notes.