It seems that I am discussing graphical design while you are discussing CSS anarchism and worst-case technical details. I mentioned text paragraphs because they are the most common and traditional type of block, but images and frames are popular since at least the middle ages and they should have about the same margins as text paragraphs (solid dark objects more, transparent images with internal empty space that adds to the "official" margin less).

The components around the slider are not "unknown": it's enough to be sure that, for example, their margins are between 20 and 30 px so the slider is allowed to claim -5px off a collapsed margin without collisions.