This example fills the browser viewport with a single rounded rectangle rendered in SVG. The visualization is a blank canvas, created by selecting the `svg` element and setting its width and height to match the document body’s client dimensions, then appending a `rect` with a corner radius. The code uses D3 v5’s `select` function to manipulate the SVG and relies on CSS to fix the body’s position and hide overflow, ensuring the canvas spans the entire screen.