[css3-ui] Proposal: CSS autofill psuedoclass

Most browsers offer a feature to save and automatically fill web passwords.
 Some of the associated rendering engines -- WebKit and Presto, for example
-- style such autofilled fields distinctively.  WebKit, in particular, sets
a yellow background color (and a black foreground color).

Many web authors (source: [1], numerous StackOverflow posts) would like to
be able to override this styling.  We propose exposing an 'autofill' or
'autocomplete' CSS pseudoclass to allow authors to style such fields: see
[2].  In fact, WebKit already does so, via the "-webkit-autofill"
pseudoclass.  This was originally meant to be an irrelevant implementation
detail, as the only styles it added were flagged with "!important"; but
authors are already using this pseudoclass, coupled with JavaScript, to
override styling for such fields: [3].

We are open to alternative suggestions, in place of using a pseudoclass.
 However, we believe that reaching some consensus here will help make the
web platform that much better.

Note that we do not believe CSS styling of autofilled fields to be a
security concern, as websites can already use JavaScript to circumvent the
rendering agent-imposed style.

[1] https://crbug.com/46543 <http://crbug.com/46543>
[2] https://bugs.webkit.org/show_bug.cgi?id=66032
[3] https://crbug.com/46543#c22