@sun @pipivovott haven't dealt with webps but for apngs:

[bocchi:~] % cat Documents/Creativity/Emoji/utils/apng_to_gif.bash
for i in *.png; do
frames=`identify APNG:$i | wc -l`
if [ "$frames" \> "1" ]; then
echo $frames
mkdir -p trash
convert APNG:$i "${i%.png}.gif"
mv $i trash
fi
done