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

I think you might want to make this -moz-margin-start instead of padding, and it seems to me that it ought to be .6rem instead of .5rem.  You've got .4rem for the watched indicator, then 21rem for the thumbnail. And you're setting the width of the details to 100%-22rem. So there is still .6rem unaccounted for.  If this line is padding, then it is part of the width of the details (meaning part of the 100%-22rem). If it is margin, then it fills in the gap between the details and the thumbnail.