How is this animated .png possible?

Question

While learning how to edit Firefox themes, I came across this animated .png (not apng) that I do not understand. I assumed it was a gif at first just given the pixel art style.

.png in question:

(reuploaded to a site of mine)

The .png is part of this theme:

• https://addons.mozilla.org/en-US/firefox/addon/black-rain-remasterd-dark-mode/

and this is the url for the image in question when I inspect the theme:

• moz-extension://0c38fbf3-159e-4a75-8215-b2e881443fc9/images/0.png

Tried to open in Photoshop and a visual studio code, opens like any other still .png would and not like say a .gif that would show me the frames.

Answer 1

It is a valid, animated PNG.

You can check it with pngcheck as follows:

pngcheck -v 0.png

File: 0.png (991263 bytes)
  chunk IHDR at offset 0x0000c, length 13
    640 x 65 image, 8-bit palette, interlaced
  chunk PLTE at offset 0x00025, length 768: 256 palette entries
  chunk tRNS at offset 0x00331, length 256: 256 transparency entries
  chunk pHYs at offset 0x0043d, length 9: 3779x3779 pixels/meter (96 dpi)
  chunk acTL at offset 0x00452, length 8
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fcTL at offset 0x00466, length 26
    unknown private, ancillary, unsafe-to-copy chunk
  chunk IDAT at offset 0x0048c, length 21371
    zlib: deflated, 32K window, maximum compression
    rows per pass: 9, 9, 8, 17, 16, 33, 32
  chunk fcTL at offset 0x05813, length 26
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x05839, length 21371
    unknown private, ancillary, unsafe-to-copy chunk
  ...
  ...
  ...
  chunk fcTL at offset 0xecc09, length 26
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xecc2f, length 21468
    unknown private, ancillary, unsafe-to-copy chunk
  chunk IEND at offset 0xf2017, length 0
No errors detected in 0.png (100 chunks, -2282.8% compression).

Or check with:

magick identify -verbose 0.png

---

You will see it contains both fcTL "Frame Control Chunk" and acTL "Animation Control Chunk". See here.