A vertical gradient of colored rectangles fills the screen, with hues shifting from dark to light across the horizontal axis. The visualization uses D3.js (`d3.select`, `d3.range`) to generate and position the rectangles, each with a fixed width and a computed RGB fill. A circular mask, created with `d3.symbol` and applied via SVG `mask`, reveals only the central area of the gradient, leaving the rest dimmed. The code sets up an SVG canvas sized to the browser window and animates a smooth transition on the body element over 10 seconds.