This example generates a grid of horizontal and vertical rectangles using D3’s `select`, `range`, and `join` methods, rendered as SVG. The full screen is filled with thin red lines, while two circular masks—one inverting the other—reveal a solid white circle at the center. The masks are built with D3’s `append` and `attr` calls, creating alternating visible and hidden regions. The code uses D3 v6 and runs entirely in the browser without external data, relying on window dimensions to size the composition.