A blank, full-viewport SVG canvas is created, filling the browser window with a single rounded rectangle. The example demonstrates a minimal starting point for a visualization, with no data or marks drawn. The code uses D3 v5’s `select` function to target the SVG element, sets its dimensions to the document body’s client width and height, and appends a `rect` with a 40-pixel corner radius, all styled by a fixed-position CSS layout that removes margins and scrollbars.