Seems this is widely accepted by browsers
https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats
ffmpeg -i test.mp4 \
-c:v libvpx -crf 10 -vf scale=640×480 -b:v 512k \
-c:a libvorbis -b:a 128k -q 10 \
-pass 1 \
-passlogfile abcd \
-y test-vp8-vorbis.webm
ffmpeg -i test.mp4 \
-c:v libvpx -crf 10 -vf scale=640×480 -b:v 512k \
–c:a libvorbis -b:a 128k -q 10 \
-pass 2 \
-passlogfile abcd \
-y test-vp8-vorbis.webm