A blank SVG canvas is filled with a rounded rectangle that resizes to fit the browser window. The example demonstrates fundamental DOM manipulation using D3.js v5, showing how to select an SVG element, set its width and height attributes, and append a styled rectangle shape. The code uses `d3.select` to target the SVG, then chains `.attr` calls to configure dimensions and corner radius, with CSS fixing the body to prevent scrolling. This is a minimal example intended to illustrate basic D3 selection and element creation.