Hydration refers to the client-side process during which Vue takes over the static HTML sent by the server and turns it into dynamic DOM that can react to client-side data changes.
Since the server has already rendered the markup, we obviously do not want to throw that away and re-create all the DOM elements. Instead, we want to “hydrate” the static markup and make it interactive.
In development mode, Vue will assert the client-side generated virtual DOM tree matches the DOM structure rendered from the server. If there is a mismatch, it will bail hydration, discard existing DOM and render from scratch. In production mode, this assertion is disabled for maximum performance.
One thing to be aware of when using SSR + client hydration is some special HTML structures that may be altered by the browser. For example, when you write this in a Vue template:
The browser will automatically inject
<table>, however, the virtual DOM generated by Vue does not contain
<tbody>, so it will cause a mismatch. To ensure correct matching, make sure to write valid HTML in your templates.