This example demonstrates masking in SVG by creating a grid of horizontal and vertical rectangles, each with a mask applied. Two masks are rendered using D3's `symbol` API, placing geometric shapes like circles and crosses across the canvas. One mask is inverted by using black as the background and white for the shapes, while the other uses the opposite color scheme, causing the grid lines to appear only within the symbol outlines. The visualization shows how D3 selections, joins, and the `attr` method dynamically set mask references and symbol paths.