A white circle follows the mouse cursor across a solid blue-gray background. The example uses D3 v7 to select the `#app` container, append an SVG canvas sized to the browser window, and create a single circle element. A `mousemove` event listener updates the circle’s `cx` and `cy` attributes with the event coordinates, providing direct, real-time feedback. The visualization is rendered entirely through SVG, with styling applied via an external CSS file.