Burn .ass subtitle onto .webm with no loss of quality
Use

ffmpeg -i input.webm -vf ass=subtitles.ass -b:v 0 -crf 20 output.webm


The libvpx encoders for WebM, by default, perform Constrained Quality encoding, and use the bitrate as a ceiling. Default bitrate is 200 kbps. To remove bitrate ceiling, set bitrate to 0. -crf controls the quality level.