--alpha=yes doesn't affect only transparent videos and images, but also the background. I spent time researching how to implement transparent backgrounds and had no idea that they already worked at least on Wayland. Background transparency will work on X11 when https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2376 is merged. It doesn't work on Windows. No idea about macOS. Either way, this paragraph already says that it only works on certain environments twice. References #6590