APNG files are valid PNG files. When an APNG file is viewed in a program that does not support the APNG extensions, it will appear as a still image chosen by the creator of the APNG file.
APNG supports 24-bit color and 8-bit transparency, whereas GIF supports 256 colors per image palette and binary transparency.
APNG is supported on Mozilla Firefox (Gecko layout engine) as of version 3.0 (June 17, 2008) and Safari (WebKit layout engine) as of version 8.0 (October 16, 2014). It's also supported on Firefox for Android and iOS Safari.
Blink layout engine based browsers like Google Chrome and Opera (post version 15) require an add-on to render APNG files.
It used to be supported natively on Opera (Presto layout engine) from version 9.5 (June 12, 2008) through version 12.17 (April 23, 2014).
Microsoft Internet Explorer (Trident layout engine) does not support APNG.