Something to keep in mind though : transparency in videos won't work.
We are limited by the technologies implemented by the web browser vendors. WebM (the open video format pushed by the Mozilla Foundation in Firefox and Google in Chrome) allows video with transparency, but it is not supported by Safari nor IE / Edge. There is no good cross browser technology available to us at the moment.
If you upload a video with a transparency layer, the transparent pixels will be rendered solid black in Previz.