Hydration

Hydration is the process by which a client framework takes over already-rendered HTML: it walks the existing Document Object Model, attaches event handlers, and initializes its internal state so subsequent updates work — without discarding and re-creating the markup.

Its defining constraint is that the client's first render must match what the server produced. Where they differ you get a hydration mismatch, which frameworks report as an error and resolve by discarding the server's work. The usual causes are all forms of the same thing: the two environments disagreed about something. Current time. A random value. A user preference read from the browser. Anything derived from window.

Hydration is also the main cost of Server-Side Rendering as usually practiced: the framework and every interactive component must be downloaded and executed before the page responds to input, so a page that looked ready is not. That gap is what Islands Architecture, React Server Components, and partial hydration all exist to shrink.

A subtler hazard: content rendered by the server and then mutated directly by the browser — by a text editor, an embedded map, a third-party widget — is invisible to the framework's model. A subsequent re-render diffs against a stale picture and duplicates or destroys elements. Keying the wrapper on a value that changes with the data, forcing a clean remount, is the usual escape.