Term
Hydration
The process by which static HTML rendered on the server is retroactively given JavaScript interactivity in the browser — turning "dead" markup into a reactive app.
Hydration — explained in more detail
Classic SSR frameworks like React, Vue or Svelte deliver ready HTML on the first request — users see something immediately, LCP is good. To make buttons clickable and state work, the browser additionally loads the JavaScript bundle and “hydrates” the HTML: event handlers are attached, component state is built up, the virtual DOM is reconciled with the real DOM.
This carries a performance cost: hydration briefly blocks the main thread and can worsen INP. Modern answers are partial hydration (hydrate only interactive islands — like Astro’s Islands), selective/progressive hydration (prioritized by visibility), and resumability (Qwik skips hydration entirely).
Example / practical use
An Astro page with three <Counter client:visible /> components hydrates only those three islands once they scroll into the viewport — the rest of the static HTML stays JavaScript-free.
Distinction from related terms
SSR is a prerequisite for hydration; without pre-rendered HTML there’s nothing to hydrate (that would be CSR). Resumability (Qwik) is the counter-position: resume serialized state instead of rebuilding it.
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