For framed ButtonElement:
In Chrome, 2px above, 4px below:
In Firefox 2px above, 5px below:
This issue is compounded by the fact we aren't sure how tall the ButtonElement is as the browser may or may not be applying the min-height: 2.5em rule, which doesn't play nicely with border-box.
For the other widgets relying on em sizing is unreliable due to different fonts used as sans-serif default.