Converting from Gif to WebM in Q mode outputs an incomplete video when targeting low CRF values. For example, running the command below produces a WebM video with a duration of 1.15 secs while the input Gif is 2.16 secs.
ffmpeg.exe -i in.gif -c:v libvpx-vp9 -b:v 0 -crf 10 -an -f webm out.webm
Targeting lower quality (e.g. CRF=~30), will produce the right and complete video output.
