CSS's shorthand order — (all), (vertical, horizontal), or all four
clockwise from the top — because that is the order every author already
knows, and none of the four is more fundamental than the others.
Distance from the bottom edge.
Distance from the left edge.
Distance from the right edge.
Distance from the top edge, in layout units.
left + right — the horizontal space these insets consume.
top + bottom — the vertical space these insets consume.
Symmetric edge insets in layout units — padding and margins (§74).
A mutable class rather than an object literal, for plan D7's reason: insets are read every layout pass, so they are written in place (
set,setAll,copy) and never replaced.