As a phoneposter, I use Inshot for video editing - just adding music to album art and saving it in 360p - and To mp4 video converter by Clogica for webm conversation. When converting to webm, I'll set the video quality to Medium and the frame rate to 1 fps. I don't mess with the audio, which had a default bitrate of 160 mbps. Since the video is only playing at 1 frame per second, it takes less than 10 seconds for the video to be converted under 6 megabytes.
I haven't messed with Webm for Retards on PC in a while, since I find it easier to use my Android for this shit.