mw.util uses much code and browser sniffing to display the access key in the tooltip. With HTML5 there is a property to simplify this, see .
Using it when available has the advantage that you don't have to figure out the correct prefix yourself, additionally it will be localized.
In Firefox 10 it works as expected:
The only drawback is that you still must provide a fallback for browsers not supporting this (including IE 9).