The thing that I don’t understand is, how can the browser render the SSR content if the JS bundle is render-blocking? Doesn’t the JS bundle prevent anything from rendering until all its JS has been executed?

It's only render-blocking if put before the content! You can actually see this if you load up the demo[0] and reload the page. For some reason, I've made the mistake of putting the script-tag for the JS bundle _after_ the main content but _before_ the footer. The footer is also rendered on the server, but you'll see - especially if you throttle your connection - that the main app will appear instantly, but the footer ("Try without javascript :)", etc.) won't appear until the JS has finished loading.

This is why today it's common to put script-tags for external JS at the end of the body-tag.

[0]https://todo-react-redux-noscript.herokuapp.com/

Thanks. That explains it. I wasn’t sure if <script> at the end of <body> blocks rendering of the preceding content. Apparently not.