A blank canvas fills the browser window with a single rounded rectangle, demonstrating the simplest possible SVG rendering with D3.js. The visualization uses the `select` function from D3 to target the SVG element, then dynamically sets its width and height based on the viewport dimensions. The code appends a `rect` element with rounded corners (`rx` attribute) that covers the entire canvas, creating a subtle frame effect. The page layout is controlled with fixed positioning and hidden overflow in CSS, ensuring the canvas remains perfectly sized to the viewport without scrollbars.