This example displays a choropleth map of the United States with counties rendered as SVG paths, filled in light gray and highlighted in light blue on hover. The code loads a TopoJSON file of US counties from a remote GitHub repository and uses `d3.geo.mercator` with a fitted projection to size and position the map within the viewport. Counties are drawn via `d3.geo.path`, and the script references an additional tooltip feature using `d3.tip` in the accompanying test.js file, though that tooltip is not implemented in the visible code. The map is rendered entirely with D3 v3 and SVG.