This example shows a blank, rounded rectangle filling the browser window as a simple canvas. It demonstrates fundamental DOM manipulation and SVG creation using D3.js v5. The code calculates the width and height from the document body, sets the SVG element's size accordingly, and appends a rectangle with a defined corner radius. Using the `select` function and chained `attr` and `append` methods, it illustrates how to build and style SVG shapes entirely through D3’s programmatic interface.