But the performance is so bad it isn't worth using. You're better off encoding a double height AV1 and mashing it together with WebGL https://jakearchibald.com/2024/video-with-transparency/. That's how bad the situation is.

Is there an advantage to this as a separate layer compared to just having an alpha channel baked into the image data itself? If not premultiplied then it would not affect the other channels either.