This example creates an animated SVG grid of horizontal and vertical rectangles, revealing a circular aperture at the center through the use of D3.js masks. The code uses `d3.select`, `d3.range`, and `d3.join` to generate 100 rects in each direction, with two distinct mask groups to alternate light and dark circle patterns. A repeated `window.setInterval` triggers an alert dialog, interrupting the visual. The composition relies on SVG `<mask>` elements to blend the overlapping lines.