I've just found out that IE 11 & 12 can't handle border-radius properly. Sometimes (not always, what is extra wierd) there is a 1px gap between the border element and the next element like here
When I drop the border-radius all is fine. Anybody knows a solution/workaround for this?
Yes this is a bug which has been reported in IE11. You can refer IE 11 Div With "border-radius" Bug which contains the solution and the discussion on the same.