This example renders a single line of large, centered text that reads “Hello React!” in the browser. It demonstrates the fundamental structure of a React application by creating a functional `App` component, which returns a `<div>` element with a CSS class. The component is mounted into the DOM using `ReactDOM.render`. The code relies on React 16.13.1 and ReactDOM from the UMD production builds, loaded via external scripts, and the styling uses a simple global CSS rule for the `.message` class.