Button link focus styling doesn't override inherited visited link styling

[craigcook]

Description

When a link is styled as a button it can take on an additional :visited state that button elements don't get. When a button link has been visited and is then focused, the regular visited link colors still apply and the button becomes unreadable.

Button focus styles should be consistent whether it's a link or a button so we may need some additional rules to handle :visited:focus.