If you want to make a video transparent right now thigs are a bit messy and its a bit hard to do. To makes matters worse Aframe version 0.9 broke alpha channel so we are stuck with version 0.8.
Here is a working example (Using Hiro marker), hope you find it useful
https://agusalex.github.io/arjs-alphachannel-video/
If you want to try out without AR you can simulate it here:
https://agusalex.github.io/arjs-alphachannel-video/arEmu.html