A grid of alternating horizontal and vertical SVG rectangles is masked to reveal a circular window at the center of the viewport. The example demonstrates how to construct and apply multiple SVG masks using D3’s selection and data-join methods, specifically `selectAll` with `range` and `join('rect')`. Two masks (`circle-mask` and `circle-mask-2`) use black-and-white rectangles and circles to control visibility of the 50 strips in each direction.