to get WebMs from YouTube

>  vp8/vorbis
youtube-dl -f43 $(xsel -b)

>  vp9/opus
youtube-dl -f bestvideo[vcodec=vp9][height<500]+bestaudio[acodec=opus] $(xsel -b)

>  to get just a part of a video without downloading the whole thing
ffmpeg -ss 100 -to 200 -i $(youtube-dl -gf43 $(xsel -b)) -c copy ytcut.webm
you get the idea