A double pendulum is drawn as two black line segments connected by a dark circle, with a white ring tracing the path of the second mass against a full-screen SVG. The animation runs on a 50-millisecond timer, updating the pendulum’s position based on trigonometric functions of a time variable. It is rendered with D3 v6, using `select`, `range`, and `join` operations to create and update the `line` and `circle` elements. The pendulum’s motion follows a classic two-arm harmonic pattern with a fixed frequency ratio.