SVG Masks
This example shows how SVG masks can punch a heart-shaped hole through a solid red circle, leaving the surrounding area visible. A `<mask>` element defines the cutout using a black rectangle for invisible regions and a white path for the visible heart shape. The mask is applied to a `<circle>` via the `mask` attribute, with the heart path drawn using an SVG arc and quadratic Bézier commands. The page is plain HTML with inline CSS and JavaScript, requiring no external libraries.