The World Countries Tree visualizes hierarchical data as a horizontal dendrogram using D3.js v5. It renders an SVG with tree links drawn as curved paths and node labels positioned by depth, with font size decreasing as depth increases. The example uses `d3.tree`, `d3.hierarchy`, and `d3.linkHorizontal`, and supports panning and zooming via `d3.zoom` and `d3.event`. Data is loaded asynchronously from `data.json`, with text styling applied via CSS.