This example shows a simple introductory D3 visualization that replaces the entire body content with the text “Hello d3 v6.” It demonstrates the most basic use of D3’s `select` method to target an element and set its text content. The HTML page loads D3 v6 from a CDN and uses a CSS class to style the message with a large, centered font. The rendering is purely text-based, with no data or graphical marks involved.