Export WebM with alpha

VP8 and VP9 support video with an alpha channel in a FFmpeg pixel format called yuva420p. I would like to export animation as a video to use with a video editor that does not support WebP, Lottie, or SVG animation, which is most. But namely for Shotcut of which I am the founder. :-) I want to recommend Glaxnimate to my users, and this will go a long way because most people want the animation to have a transparent background. All you need to do to use this is to export as video with a .webm extension. The default background color of transparent is fine for non-alpha videos: transparent black is exported as black. And if you change the color to something else and do not increase the opacity it exports as opaque in the chosen color.