A black rounded rectangle fills the entire browser viewport. The visualization adapts to the screen size by querying the document body's dimensions and setting the SVG's width and height attributes accordingly. The code uses d3.select to target the SVG element, then appends a rect with a corner radius of 100 pixels. The SVG is rendered in the browser, with the script loading d3 v5 from the unpkg CDN. The rect's dimensions match the computed viewport size, creating a simple full-screen black canvas with softly rounded corners.