A rounded, full-viewport rectangle fills the browser window, rendered as a single SVG `<rect>` with no data mapped to it. The example demonstrates a minimal blank canvas using D3 v6's `select` and `attr` methods to set the SVG's width and height from the document body's client dimensions. The rectangle's `rx` attribute softens its corners, while CSS fixes the body to prevent scrolling. The visualization shows only this simple geometric form, with no axes, marks, or interaction beyond the static shape.