How can I restore the initial value of outline for :focus?

For the record, something I found to work in Chrome, but not Firefox:

a:focus {
  outline-style: none;
}

div.where-i-want-this-style a:focus {
  outline-style: auto;
}

This works because Chrome's initial value for outline-style is pretty straightforward (auto), and setting just that part to none is enough to disable the default outline.

Unfortunately, Firefox appears to use a vastly different method for setting its default focus style. If I'm reading Firebug right, it may not be expressible in CSS, which means there's no way to get it back once it's overridden. Once I discovered that I didn't bother checking any other browsers.