Relates to MC-12699.

The bug

If an animated png file is used as a texture, the game will only render the first frame of the file, and all following frames will be ignored.

How to reproduce

A resource pack is attached that replaces the glass texture with this public domain animated PNG file: https://commons.wikimedia.org/wiki/File:Animated_PNG_example_bouncing_beach_ball.png

1. Download and apply the attached resource pack
2. Look at glass

Expected results

The texture would be animated.

Actual results

It is not.

How to fix

Much like MC-12699, this issue most likely arises from a broken or incomplete implementation of the png specification.

Whether these files should be converted over to Minecraft's native animation format is another question entirely.