Actually it was called APNG / MNG - http://en.wikipedia.org/wiki/APNG
However, it never really caught on because people seemed to wanted to use the browser non cross platform compatible plugin called Flash instead of a format that could been built into every browser for any platofrm. LOL. Oh well.
The other reason it may not have caught on was that there were very few (good) programs to create the animations themselves by comparison to the other competing formats.