after effects transparent background not working

im making vfx with saber effects and im using rgb + alpha, millions of colors+, straight(unmattered), and rendering with avi file type, then in media encoder im using the webm plugin with vp9 codec, and ensuring to check "include alpha channel" and its a black background everytime. I tried toggling transparency grid in AE to make sure its transparent and it is. i also tried the apple pro res 4444 method and it didnt work either. Any help would mean alot!