This example dynamically sizes a D3.js visualization to fit the full browser viewport. The visualization draws a single rounded rectangle that fills the entire window, with its width and height directly tied to the dimensions of the `document.body` element. The code uses D3's `select` function to grab the SVG element and append the rectangle, setting its `rx` attribute for rounded corners, while CSS positions the body to prevent scrolling. The rendering approach is purely SVG-based, with no external data source.