React example is not using it is to server render React or something like that, http4k serves it as a normal SPA after it is built from what I can see. But, it is a nice example on how to call node/yarn tasks from Gradle, so it would be useful for me. I would probably then just use Webpack to process CSS assets (with something like PostCSS) and maybe images and integrate that through Gradle in a similar way.
Hotwire examples are cool, definitely will try that