This example shows a gray inner rectangle centered within a scalable SVG container, with a ResizeObserver logging the container’s dimensions on every resize. The visualization uses D3’s `select`, `selectAll`, and `join` to bind and update SVG elements, while the `viz` function computes inner dimensions from configurable margins and sets the rectangle’s fill via the `innerRectFill` property. The code demonstrates basic responsive layout with a fixed margin structure and dynamic width and height values derived from the container’s client size.