Why is chrome highlighting a margin that is not listed in styles or computed

My guess here would be that the containing small-4 possibly has a padding? Or Chrome is trying to be smart.

This is caused by the display: block rule.

The element is occupying the entire horizontal space of its parent, so Chrome tries to convey that by coloring the occupied space with the margin color.

Change the rule to either display: inline-block or display: inline to remedy the issue.