If you're here, you're probably wondering "why did the file I downloaded have a black background?" As someone who doesn't really know much of anything about video encoding, I'll do my best to explain what's going on in a way that hopefully won't make your eyes bleed.

After I export the .apng file from Spine, I convert it into .webm with ffmpeg. But it's not as simple as me just putting the file in and then getting a file out--there's a bunch of settings I've messed with in order to try and get the videos compressed while still looking alright.

Thes are the specific commands I use (note that I'm using Windows; if you're on a Mac or Linux-based machine, your command will look a little different):
.\ffmpeg.exe -i in\$file.png -c:v libvpx-vp9 -b:v 0 -crf 30 -pass 1 -pix_fmt yuva420p -f null NUL
.\ffmpeg.exe -i in\$file.png -c:v libvpx-vp9 -b:v 0 -crf 30 -pass 2 -pix_fmt yuva420p out\$file.webm

The things that are of primary interest are the codec I'm using (libvpx-vp9) and the picture format I'm using (yuva420p). I'm not certain if many video players support these formats, and I know for a fact that mpv doesn't really like transparent videos in general.

If you download the videos, and, upon playback (within a browser), they still have a black background, I unfortunately don't really know what the solution is to that yet. But—if you would like to extract the frames transparently, you can use ffmpeg, again. This time, I recommend:
ffmpeg.exe -c:v libvpx-vp9 -i $file.webm -vf fps=60 -pix_fmt rgba out\frame%03d.png

This should give you each frame of the video, transparent.

Of course, for cookies that the video encoding wasn't as kind to, such as Beet or Madeline, this won't fix the compression present on certain parts of their sprite. If you would like the full versions of their animations, please email me at the email listed on my FAQ page, and I'd be happy to provide the .apng files. Extracting all the frames from those using ffmpeg is a far easier process.