This example demonstrates the minimal setup required to render a React component in a web page. It displays the text “Hello Reaction !” as large, centered content within a styled div. The code uses `React` and `ReactDOM` from the React 16.13.1 UMD builds, with `ReactDOM.render()` attaching the `App` component to the `#root` element defined in the HTML. Styling is applied via inline CSS in the `<style>` block, targeting the `.message` class. The bundled output runs from `bundle.js`.