A smiley face rendered as a full-screen SVG using React components. The face consists of three circles—a yellow head with a black stroke, and two black eyes—positioned relative to the center of the viewport. The code uses React 16.9 with `ReactDOM.render` to mount the app, and computes circle coordinates from `window.innerWidth` and `window.innerHeight`. All dimensions, including eye offsets, eye radius, and stroke width, are defined as constants in JavaScript.