ButtonWidgets are currently not applied box-sizing: border-box. In contrast to several other widgets and even mediawiki.UI.
Default box model sizing is nerve-wracking and we are already starting to overwrite OOjs UI's default in prominent places
IE 6 & IE 7 are the only mention-worth browsers that don't support it, but buttons in general still stay usable.
Buttons with icons and indicators seem to be the elements to be careful about.
Let's better change the default…