This example displays a single circle centered within an SVG canvas, sized to half the height of the 960×500 viewport. It demonstrates minimal D3.js v5 usage, where the `select` function targets the existing SVG element and `append` creates the circle shape. The circle’s radius, x-coordinate, and y-coordinate are set via the `attr` method, using the hard-coded dimensions for height and width. The page is styled with CSS, setting a monospace font and removing margins, while the circle itself is rendered with default black fill.