This example demonstrates how SVG masks can be used to create geometric cutouts in layered grid patterns. It renders 100 horizontal and vertical rectangles across the full viewport, each masked by a circle that reveals or hides portions of the grid. The code uses D3.js v7 with the `select`, `selectAll`, and `range` APIs to generate the shapes, while two separate `<mask>` elements with contrasting fill colors control visibility. A white-filled mask with a black circle creates the first cutout, and an inverted mask creates the second, producing two distinct circular viewing windows.