[css-ui] outline-stye: auto

At the Web Platform Doc Sprint in Amsterdam on October 12th, we were
documenting the new *auto* value for the *outline-style* property,
described in the CSS Basic User Interface Module Level 3 (CSS3 UI) [1].

This is how we resolved to describe it in the Web Platform Documentation
[2]:

*outline-style: auto*

Defined by the user agent (browser). Currently supported only in WebKit.
(Seems to map to the keyboard-focused link style, at least on Chrome for
Mac. At the Amsterdam doc sprint in October, 2013, we tested this on
several browsers, without being able to discern a common implementation.)
The specification reads as follows: "The ‘auto’ value permits the user
agent to render a custom outline style, typically a style which is either a
user interface default for the platform, or perhaps a style that is richer
than can be described in detail in CSS, e.g. a rounded edge outline with
semi-translucent outer pixels that appears to glow. As such, this
specification does not define how the ‘outline-color’ is incorporated or
used (if at all) when rendering ‘auto’ style outlines. User agents may
treat ‘auto’ as ‘solid’."

This description, besides being laughably verbose, points out a clear
problem with the spec: it does not specify a operand to which the
*auto*value resolves.

Presently there is nothing to determine how the auto value should be
implemented, and the specification needs to be, well, more specific.

Thanks!
~ Your admirers at Web Platform Docs

[1] http://dev.w3.org/csswg/css-ui/#outline-style0
[2] http://docs.webplatform.org/wiki/css/properties/outline-style