A blue div demonstrates chained D3 transitions, changing background color to green, fading to transparent yellow, then returning to full opacity over three 3-second stages. Using D3 v7’s `transition`, `duration`, and `style` methods within `index.js`, the animation runs on page load via `main()`. The example shows basic sequencing without data, relying on SVG-free DOM manipulation and the `d3.select` API.