Derk-Jan filed a bug (https://bugzilla.wikimedia.org/show_bug.cgi?id=62924) about the focus state for buttons. There is also a patch allowing the browser defaults at https://gerrit.wikimedia.org/r/#/c/119998/
I was also somewhat concerned about this. As I recall (I think we talked about this at one of the MediaWiki UI hack days), we decided to tentatively go forward with hover == focus == bevel, but I think people were open to revisiting it.
At any rate, I think he's right, and we should come up with a better focus styling, either a custom one (probably better, but it should be more distinctive than the current one, both from hover and from normal state) or allowing the browser default.
Matt Flaschen