This example shows a hierarchical tree visualization of world regions and countries, with text labels sized inversely to their depth in the hierarchy. The layout uses `d3.tree` and `d3.hierarchy` to position nodes and links, rendered as SVG paths via `d3.linkHorizontal`. Pan and zoom interactions are enabled through `d3.zoom`, applied to a containing group. The data comes from a nested JSON file, and the visualization fills the browser viewport by reading `document.body.clientWidth` and `clientHeight`.