[css3-ui] ID selectors as values of nav-{up,right,down,left} properties

Hi,

While reviewing the CSS3 UI module, we noticed that the nav-{up,right,
down,left} properties take ID selectors as values.

                http://dev.w3.org/csswg/css3-ui/#nav-dir

As an author, if I encountered

#foo {
  nav-right: #bar;
}

in a stylesheet, I would just assume that I could do something like

[type=submit] {
  nav-right: [type=reset];
}

but CSS3 UI doesn't allow this (thank $DIETY). Generally speaking, this
Working Group has avoided allowing selectors as property values in other
places, and I'm reluctant to make an exception here.

Thoughts?


Ted

P.S. A relevant WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=66027