This example displays a blank, full-window SVG canvas with a single rounded rectangle filling the viewport. The code uses D3 v5's `select` function to target the SVG element and sets its width and height attributes based on the browser window dimensions. A `rect` element with a corner radius of 40 pixels is appended and sized to match the available space. The minimal setup demonstrates how to initialize a D3 visualization with dynamic sizing and CSS styling that locks the body to the viewport.