Why doesn PNG support animation?
The answer is either “it does” or “because it’s a bad idea,” depending on your perspective. The animation-supporting version of PNG is called MNG, for Multiple-image Network Graphics. For most practical purposes, MNG is PNG, just with a different filename extension, a slightly different file signature, and the bare minimum of internal changes necessary to support animations (and other forms of multi-image files). It has exactly the same chunk structure as PNG and even shares most of the same chunks; the major difference is the addition of new chunk types to support features such as looping, clipping, and so on. The majority of the PNG developers felt (and still feel) that overloading a single file type with both still and animation features is a bad design, both for users (who have no simple way of determining to which class a given image file belongs) and for web servers (which should use the image/foo MIME type for stills and video/foo for animations–GIF notwithstanding). Programmer
