the concept is not new in an sense (duh), I mean, send HTML first send the important CSS first so the browser can show something - and with the correct dimensions hopefully to avoid reflows (and flash of white and so on), and therefore JS interaction "always" got enabled on some kind of trigger (onload, DOMContentLoaded, or simply it the script tag was added after the closing body tag, which as far as I know led to all of the event handlers registering by the time the DOM tree was there)

then chunking was the next step, and basically then logical endpoint is this mix-and-match strategy that NextJS is "leading" (?), by allowing things to be streamed in while sending and caching as much of the static parts up front as possible.