BlankCanvas fills the browser viewport with a single rounded rectangle, using the full window dimensions captured from `document.body.clientWidth` and `document.body.clientHeight`. The example demonstrates a minimal SVG setup where D3 v5 selects the `<svg>` element and appends a `<rect>` with a 40-pixel corner radius. The rectangle remains static, illustrating basic attribute assignment and responsive sizing without interaction or data binding. Styles in CSS keep the page fixed and hide overflow, ensuring the shape always matches the visible area.