Hydration

Also called rehydration, client hydration

The step where a framework attaches event handlers and state to HTML that arrived already rendered. The server sends complete markup, the browser displays it immediately, then JavaScript loads and takes over the existing DOM instead of rebuilding it.

For crawlers this is mostly a non-event. The content was in the response body, so a crawler that never runs a line of JavaScript still read the page. Hydration is about interactivity, not visibility, and confusing the two is where the SEO advice about it goes wrong in both directions.

The mistake

Assuming the server HTML is what stays on the page.

A hydration mismatch is the client rendering something different from what the server sent. Common causes are a formatted date, a random identifier, a value read from localStorage, or a branch on window width. React and similar frameworks warn in the console and, in the worst case, discard the server markup and re-render from scratch. The visible symptom is a flash. The invisible symptom is that the content a crawler read is no longer the content on the page.

The second failure is arranging for content to arrive only after hydration. A component that fetches its data on mount produces server HTML containing a spinner, which is exactly what a non-rendering crawler stores. That is a JavaScript rendering problem wearing a different name, and it is invisible in a browser because the fetch completes in 80 milliseconds.

Hydration also costs the user. Shipping and executing the JavaScript for a whole page delays interactivity even though the pixels are already there, which is why partial hydration and islands architectures exist. That cost sits after time to first byte rather than inside it, so a fast server response can coexist with a page that ignores clicks for two seconds.

The check is the same one as always. Compare the raw HTML with the rendered DOM, and treat anything that exists only in the second as content most crawlers will never see. Prerendering narrows the gap; it does not close a mismatch.