A single large rectangle sits against a plain background, initially drawn in dark red before transitioning to a bright blue over three seconds. The animation uses a D3 transition with a bounce easing, applied to the rectangle’s fill style. The visualization is built with D3.js (version 3.5.5) and rendered as an SVG element that fills the viewport. The code originates from a Blockbuilder.org gist and demonstrates a minimal, animated color change without any data or interactivity.