Video isn't Transparent

[lightandlense]

I guess I'm a little confused because the even though I'm positive that the video is transparent (I tested it online to make sure) it does not display that way. Even in your example it's a grey background and not a transparent background so I'm not sure how it all works.

I tested to make sure it was transparent here: https://rotato.app/tools/transparent-video
Screenshot listed below.

When using the code however I still get a black box around the subject second screenshot listed. Suggestions?