A full-window SVG canvas presents a single rounded rectangle on a blank page. The rectangle is sized to match the browser viewport dimensions, which are read from `document.body.clientWidth` and `document.body.clientHeight`. The code uses D3 v5's `select` to append the shape, setting its corner radius with `attr('rx', 40)`. The example demonstrates a minimal setup for creating a responsive, full-screen visual placeholder using D3 and SVG, with CSS positioning the body to fill the viewport.