User @laurentdesserre successfully used a simple http link to a .gifv file in a video widget to display a video. It works upon publishing and we know why: our modern browsers are smart and can play a .mp4 or .m4v file renamed in .gifv.
However Webflow seems to be fooled by it and won't display it as a video.
For reasons I don't fully understand, .gifv format is rising and if it's not too much to ask, Webflow could support it in the designer.
Thanks for listening! (and thanks for the cursors!)