I believe there is a problem with Gimp's WebP animation output. It's a hit or miss when upload to FireFox and it reports errors when imported into ffmpeg.
[webp @ 0x55a7067f2980] skipping unsupported chunk: ANIM
[webp @ 0x55a7067f2980] skipping unsupported chunk: ANMF
Last message repeated 17 times
[webp @ 0x55a7067f2980] skipping unsupported chunk: XMP
[webp @ 0x55a7067f2980] image data not found
[webp_pipe @ 0x55a7067f0740] decoding for stream 0 failed
[webp_pipe @ 0x55a7067f0740] Could not find codec parameters for stream 0 (Video: webp, none):
unspecified size
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, webp_pipe, from '/home/racer/Downloads/Gimp-versions/Frog.webp':
Duration: N/A, bitrate: N/A
Stream #0:0: Video: webp, none, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream mapping:
Stream #0:0 -> #0:0 (webp (native) -> gif (native))
Press [q] to stop, [?] for help
[webp @ 0x55a706848fc0] skipping unsupported chunk: ANIM
[webp @ 0x55a706848fc0] skipping unsupported chunk: ANMF
Last message repeated 17 times
[webp @ 0x55a706848fc0] skipping unsupported chunk: XMP
[webp @ 0x55a706848fc0] image data not found
Error while decoding stream #0:0: Invalid data found when processing input
Cannot determine format of input stream 0:0 after EOF
Error marking filters as finished
Now, if I exported and encode with ffmpeg to webp, no such errors occur. Not sure why??