This example shows a blue square rendered within an SVG element, with a circular cutout revealing the page background. A mask defined in SVG `<defs>` uses a white rectangle and a black circle to hide the circle’s area. The page uses plain HTML and SVG’s built-in mask API, with no external JavaScript libraries. The SVG is 200 by 200 pixels, and the mask applies directly to the rectangle’s `fill` via the `mask` attribute.