This example renders a full-screen smiley face using React and SVG. The face is centered on the window and consists of three circles: a yellow head with a black stroke, and two smaller black eyes positioned symmetrically. Sizing and positioning are computed from `window.innerWidth` and `window.innerHeight` in React, with the SVG root element and circle elements defined via JSX. The code uses React 16.9 and ReactDOM to mount the component into the `#root` div.