don't we want the overlay to be over the whole video ? I would have used width:100%, height:100%, left:0, top:0.

to be fair I don't exactly know what is requested here. Should we display the video and make it possible to play it ? Should we display the first frame for example ?

I just got an advice from one of our video engineer, and here is how we could get the first frame:
* create a video element under the hood
* set its src
* set a "progress" event handler
* call load
in the "progress" event handler :
* if the property `mozPaintedFrames` is > 0 then we have at least one frame (see https://developer.mozilla.org/en-US/docs/DOM/HTMLVideoElement)
* draw the video in a canvas, get the blob url from the canvas, put it in an image
* set its src to ''
* call load again (this should stop the previous load and free up resources)

My colleague told me this would probably won't work at the first try so don't hesitate to ask :)