At one point, I made an attempt to fix the focus indicator outline on

buttons, but my fix was incomplete. I had specified an outline color and

offset, but not the style, and this prevented browsers from displaying

anything for the outline.
We're now just removing our outline definition entirely, using the

browser's native style instead. There's on exception in that we're

keeping a 2px gap between the outline and the button, preventing it from

overlapping borders.
There's also a fix for focus indicators in button groups. We now set a

z-index: 1, which should let it sit just above siblings in the same

container, helping the foucs indicator stand out.

Tested focus indicators in Safari, Chrome, and Firefox.