This example presents a blank, rounded-rectangle canvas that fully fills the browser viewport. It demonstrates a minimal setup for a D3.js visualization using the `d3.select` API to target the SVG element and set its dimensions based on the document's client width and height. The code then appends a single `rect` element with rounded corners (`rx` attribute), whose size matches the viewport. The visualization is rendered as a static SVG using D3 v5, with CSS positioning that fixes the body to the screen edges and hides overflow.