A React application renders a yellow smiley face that fills the browser viewport, adapting its size and position to the window dimensions. The face uses SVG circles for its head, eyes, and mouth, with the head positioned at the window’s center and its radius based on the viewport height. The code employs React’s `ReactDOM.render` to mount the component and React elements to define the scalable vector graphics, with the smiley’s features offset from the center using specified eye and mouth coordinates.