What is auto outline in Webkit?

What does auto mean when I write:

:focus {
    outline: auto 5px -webkit-focus-ring-c开发者_运维百科olor
}

It is not documented here and I cannot find documentation elsewhere.

You're using the shorthand for the outline-* properties; auto represents the value for outline-style and auto itself means that it's up to the browser to decide what to do based on the context of the element.

I would just like to point out that in IE auto doesn't show anything, and you'll have to explicitly declare what type of border you want to use. Alex K's link is a good demonstration of how other popular browsers work.