Take an 8-second clip, cut the person out with the video background removal in ImgIng, export a transparent WebM, convert it for an editor, and the background comes out black. The first suspect is the export. In this case it wasn't the export: the black came from a later step. Below are three checks to run in order. Everything here is from one clip (540×960, 2,394,232 bytes), one machine, one run, and an FFmpeg 7.1 build. I did not try other FFmpeg versions, and I found no official import notes for any editor, so I say nothing about how editors treat the file.

Check whether the file really has alpha

The obvious first move is ffmpeg -i and a look at the metadata. It prints alpha_mode : 1 , which looks like proof. It isn't. When I re-encoded the clip to plain yuv420p with FFmpeg as a test, the output still carried ALPHA_MODE : 1 (FFmpeg copies the stream tag over), and it had no alpha data at all.…