You certainly don't need JSX for React, nor a build chain. My ship investor Kelly trainer game[1] is a simple React application in a single HTML file. The initial page load is slow because I experimented with using built-in browser module support to pull in React, but if you change it to serve a pre-built library from a CDN instead then also the initial page load is fast.

You can view the source on that page to see how simple it can be, but here is the essence:

1. Import React. This is the step you can do from a CDN instead of you want a faster initial page load. (Okay apparently I'm using Preact here, but it's effectively the same thing.)

    import { h, render, createContext } from 'https://unpkg.com/preact@latest?module';
    import { useState, useMemo } from 'https://unpkg.com/preact@latest/hooks/dist/hooks.module.js?module';

2. Render React components in HTML elements. This is where the progressiveness comes in. You don't have to render the entire web page as one React component; you can choose to only render some components in some locations of the page.

    const appDiv = document.getElementById('app');
    render(h(App, null), appDiv);

3. The React components are plain ES functions that can use hooks etc. from React, as usual.

    function App() {
        let [state, setState] = useState(initial_state);
        // --------------->8------
        return h('main', null, [
             h(Description, {
                 turn: state.turn,
                 strait: state.straits[state.shipment.strait]
             }),
             h(Bankroll, { wealth: state.wealth.first() }),
             h(Investment, { invest, state }),
             h(WealthHistory, { wealth: state.wealth }),
         ]);
    }

Instead of JSX we use the h function, which takes either tag name in a string, or another React component, takes an attribute dictionary as the second argument, and a list of children as the third. That's it. Super easy.

[1]: https://xkqr.org/ship-investor/ship-investor.html

(There is at least one bug in the game logic that prevents this game from being any fun at the moment. I have not taken the time to investigate why that happens, because I have not offered the training that used this tool to anyone in a few years.)

((If you enjoyed the game anyway and want more of a challenge, you might want to try the continuous version: https://xkqr.org/ship-investor/ship-investor-2.html. There is also a secret third version simulating futures which I have never shared with anyone before. It is playable, but not finished: https://xkqr.org/ship-investor/ship-investor-3.html))