This example demonstrates a minimal React application that uses D3.js to render a simple heading. The `App` component returns an `<h1>` element displaying "Hi," which is mounted into the `#root` div via `ReactDom.render`. The page loads React 18, ReactDOM, and D3 v7 from CDN links, along with custom CSS and JavaScript files. No D3-specific visualizations or data bindings are implemented, only the basic integration of React with D3's library available in the browser environment.