Using code very similar to this:
(Single embed perms, can’t attach image)
border-width: 5px 5px 5px 5px; border-style: solid; border-color: transparent Gold Gold Gold;
Results in this:
(Ignore the red, that’s the background of the parent frame)
This makes me think that borders aren’t actually “outside” the original element, but instead, the original element is extrapolated, and new “borders” placed on top of the extra extrapolated space.
If borders were actually individual pixels outside of the original element, then setting the top to be transparent (what I’ve done in the image above) wouldn’t show the upside-down trapezium of green. Instead, the red background would be shown, as the border-top is literally transparent.