Importing webm video with transparency to the VSE removes the transparency In an effort to cut down on file size, especially for archiving purposes, I've attempted to render out my old project's PNG sequences into .webm files with RGBA selected because of the transparency.  Problem is, when I load the new RGBA webm file into the Blender VSE, the alpha channel is replaced with black, and I can't seem to figure out how to get it to recognize the alpha channel.  Is this is a bug in Blender?
You can also convert the png sequence to a MPEG-4 container with a PNG codec. This will keep the transparency. The file size seems to be a little larger than the webm-codec.
The problem stays also when you convert the png sequence directly with FFMPEG (what Blender uses internally). According to this post that shouldn't be the case. However, importing that video back into Blender doesn't show - as you mentioned- the transparency.