A blank, rounded rectangle fills the entire browser window, created as a single SVG `<rect>` element. The visualization calculates the canvas dimensions from the document body's client height and width, applying a 40-pixel corner radius to the shape. The code uses D3 v5's selection API (`d3.select`) to target the SVG element and chain attribute assignments for sizing and styling. The example demonstrates a minimal SVG rendering pipeline with D3, where the resulting graphic is purely a static, empty canvas with no data or interaction.