Bug 1095826 - [RTL][Video] Thumbnail images do not appear

If the image is 21rem wide, why are you subtracting 22rem here? You've got one rem of padding on the start side below, but that counts as part of the width described here. So I think you end up with 1 rem of unused width on the end side. It that is the layout that you want, it seems like it would be easier to understand if you just made this full width (100%-21rem) and put 1 rem of padding on both the left and the right.