Trigger accesskeys on OSX in Safari, Chrome, or Firefox by holding ⌃ + ⌥ and selecting the accesskey.
For Windows in IE, Safari, or Chrome (may need shift) use alt and in Firefox use shift + alt
accesskey toggles :focus state on these buttons. Because of varying os/browser implementations and vague specs, this isn't very usable...yet. Great article/overview at webaim.org.
A Pen by Jake Albaugh on CodePen.