A blank SVG canvas fills the browser viewport, displaying nothing but a single rounded rectangle that matches the window's dimensions. The visualization demonstrates the foundational setup of a D3 project by importing the `d3` library and appending a basic shape element. Using `d3.select` on an empty SVG, it dynamically sets the width and height attributes based on the document's client dimensions, then adds a `rect` with a 40-pixel corner radius. The code relies on D3 v5 and a bundled JavaScript file, with CSS positioning the body to cover the entire viewport.