A smiley face rendered as a full-screen SVG using React. The code computes the viewport dimensions with `window.innerWidth` and `window.innerHeight`, positions the face at the center, and draws a yellow circle with a black stroke using React’s JSX syntax for SVG elements. It includes two eye circles and leaves space for a mouth, though the mouth is not drawn. React 16.9 and ReactDOM are loaded from a CDN, and the component is mounted to the `#root` element.