I have mentioned it here purescript-halogen/purescript-halogen#610 (comment)

just like for:

1. https://reactjs.org/docs/react-dom.html#hydrate
2. https://github.com/snabbdom/snabbdom/blob/master/src/tovnode.ts

The second link is just for reference. It builds VDOM from DOM node, but our hydrate method should

1. take DOM element
2. take VDOM element that contains event handlers
3. add event handlers to the DOM elements recursively AND throw error if the content of the DOM element is different from the VDOM (for example the text or class property)