What is Hydration?
In web development, hydration or rehydration is a technique in
which client-side JavaScript converts a static HTML web page,
delivered either through static hosting or server-side rendering,
into a dynamic web page by attaching event handlers to the HTML elements.
Because the HTML is pre-rendered on a server, this allows for a fast
"first contentful paint" (when useful data is first displayed
to the user), but there is a period of time afterward where the page
appears to be fully loaded and interactive, but is not until
the client-side JavaScript is executed and event handlers have
been attached.