This example demonstrates how to create geometric patterns using SVG masks and rectangles. The code generates full-screen vertical and horizontal rectangle grids, with a circular mask that reveals only the overlapping central region. It uses the SVG namespace API (`document.createElementNS`) to construct elements dynamically in vanilla JavaScript, with a mask containing a white circle to define the visible area. The rendering adapts to the browser window dimensions, setting the SVG width and height to match the viewport size.