A smiling face drawn in SVG follows the mouse cursor, with its eyes, mouth, and nose positioned relative to the window center. The React component renders the face using static JSX elements, while the translucent background circle and facial features are drawn directly with SVG primitives. The code imports React and ReactDOM to mount the app, and uses D3 v5 only for the bundled `d3.min.js` reference, though no D3 API is actually invoked. Mouse tracking is implied by the README, not implemented in the visible script.