This allows all posts - whether they are friends' updates or adverts - to take up a bigger proportion of the web browser with more prominence given to images and video rather than text describing a link.
There's still a chance we're looking at an elaborate KIRF or an early engineering prototype that could change, but given that 9to5 Mac just got very similar images with black trim, there's a real possibility that we've just been given a sneak peek of what to expect from Apple later this year.