This example demonstrates a minimal React application that renders the text "Hello React!" inside a full-screen HTML page. The code creates a single React component, `App`, which returns a `<div>` element, and mounts it to the DOM using `ReactDOM.render` targeting the `#root` div. It imports `React` and `ReactDOM` from the React library, and includes external React and ReactDOM production scripts via CDN in the HTML file. The accompanying CSS sets the body margin to zero and hides overflow.