A blank, fixed, full-viewport SVG canvas is drawn using D3 (d3.v5). The visualization only renders a single rounded rectangle covering the entire browser window, with no underlying data or marks beyond this base shape. The code uses D3’s `select` and `attr` methods to size the SVG to the viewport’s clientWidth and clientHeight, applying rounded corners via `rx`. It relies on HTML `body` CSS for positioning and uses a simple SVG `rect` element for output.