This example shows a minimal React application that renders the text “Hello React!” in a large, centered font. It uses React 16.13.1 and ReactDOM to mount a single functional component into a root div. The component returns a styled `<div>` element, with the message font-size set to 13em via a CSS class in the HTML head. The code relies on standard JSX syntax and the `ReactDOM.render` API for client-side rendering.