For those who got tired of waiting, I've added here some docs on how to cut a WebM video (like those of Wikimania 2012 on YouTube) in a few seconds for Commons upload: https://commons.wikimedia.org/wiki/Help:Converting_video#WebM_with_mkvtoolnix (Other video editors all have peculiar idiosyncrasies which were impossible to overcome for a newbie like me.) Of course you first have to download: http://rg3.github.com/youtube-dl/ Nemo