ffmpeg - Convert MP4 to WebM, poor results

How do i Convert an MP4 (H264,AAC) to WebM with zero quality loss using FFMPEG?

Converting video from 1080p to 720p with smallest quality loss using ffmpeg

Fix bad files and streams with ffmpeg so VLC and other players would not crash

// Adjust the CRF value till the quality/size tradeoff is ok. Lower values produce bigger but better files.
fmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 -b:a 128k -c:a libopus output.webm

// The default encoder for WebM, libvpx, requires that if CRF mode is used, bitrate has to be set to 0. 
ffmpeg -i lucy.mp4 -c:v libvpx-vp9 -crf 4 -b:v 0 lucy.webm

// Changing image size
ffmpeg -i MyMovie.mkv -vf scale=-1:720 -c:v libx264 -crf 0 -preset veryslow -c:a copy MyMovie_720p.mkv

//  fix video time index
ffmpeg -err_detect ignore_err -i video.mkv -c copy video_fixed.mkv

ffmpeg -crf

batch processing

for i in *.avi; do ffmpeg -i "$i" "${i%.*}.mp4"; done