A black rectangle fills the entire browser viewport. The example is a minimal demonstration of a full-screen SVG canvas. The code sets the SVG's `width` and `height` attributes to the dimensions of the document's client area, then appends a single `rect` element with a rounded corner radius of 40 pixels. It relies on D3 v5's `select` function to fetch the SVG element and chain attribute modifications, resulting in a clean, static visual that highlights the root SVG element on a fixed, overflow-hidden body.