Placeholder issue from discussions with @fantasai / @tabatkins

From our discussion varying the margins directly may not be desirable, and instead applying the adjustment like clearance. Similarly the difference between padding-box and content-box is minimal (and content-box typically being the more desirable option).

If we end up with only margin-box / padding-box could potentially change the value space to:
block-step-insert: inside for "content-box" and
block-step-insert: outside for "margin-box".