A responsive visualization container uses a ResizeObserver to track the container’s dimensions and updates the state with the new width and height. The d3-selected SVG element re-renders an inner rectangle on each resize, with its fill, margins, and size defined by the viz function. This example demonstrates a pattern for making D3 visualizations adapt to their parent element’s size changes, using the ResizeObserver API and D3’s join method to manage the SVG and rectangle elements dynamically.