Would it be possible for you to add support for the flag that tells browsers that support APNG to skip the first frame? When I try to open an APNG file as an APNG when it has that flag, it doesn't open.
This is an example of one. This image has two frames. first frame says doesn't (in red), the second says does. (in green)
If the image were to have more than two frames, APNG-aware browsers would still simply ignore the first frame and only use the ones after, as said by the flag. If I open this as a png, it opens fine, but opening it as an apng gives an error opening the file.
BTW, Something noteworthy is that APNGs (this flag included) fully work in standard base64.