I originally reported this to Epiphany but I was directed here. https://gitlab.gnome.org/GNOME/epiphany/-/issues/1654 The WebM container supports alpha channel video with the VP8 and VP9 codecs, however in Epiphany this doesn't work. Transparent video is shown with a black background instead.
The issue is with the dma-buf sink. Both the GL and software-compositing sinks render with the alpha channel blending correctly (done in https://commits.webkit.org/238313@main). To not use the dma-buf sink, set WEBKIT_GST_DMABUF_SINK_DISABLED=1. Fix for dma-buf incoming...
Pull request: https://github.com/WebKit/WebKit/pull/26598
Committed 276936@main (aff02c3a8c4f): <https://commits.webkit.org/276936@main> Reviewed commits have been landed. Closing PR #26598 and removing active labels.
<rdar://problem/125786839>