A choropleth map of the United States displays population density in people per square kilometer, using a yellow-to-red sequential color scale for each state. The map uses an Albers USA projection with `d3.geoPath`, loading state boundaries as TopoJSON from the us-atlas package. State names and density values appear as labels for the most densely populated regions, and a gradient legend at the bottom right maps color to density values. Tooltips on each state show its name and population density on hover.