GStreamer's VideoToolbox encoder recently gained support for encoding HEVC/H.265 videos containing an alpha channel.
A separate vtenc_h265a element has been added for this purpose. Assuming you're on macOS, you can use it like this:
gst-launch-1.0 -e videotestsrc ! alpha alpha=0.5 ! videoconvert ! vtenc_h265a ! mp4mux ! filesink location=alpha.mp4Click here to see an example in action! It should work fine on macOS and iOS, in both Chrome and Safari. On other platforms it might not be displayed at all - compatibility is unfortunately still quite limited.
If your browser supports this format correctly, you will see a moving GStreamer logo on a constantly changing background - something like this. That background is entirely separate from the video and is generated using CSS.