He uses mechanism that is inherent in GIF specification and can be used for animation, more than 256 color images or images that do not store image data for empty areas (or any combination). GIF stores sequence of rectangular blocks of image data each with its specified position in resulting image, own palette and optional delay before start of drawing (for animation). Browsers tend to assume that GIFs with multiple blocks with zero delay between them are broken animated GIFs (and enforce some minimum delay), because such files do exist and are certainly more common than GIFs with holes or large numbers of colors.