z-ordering of column-rules

See http://www.w3.org/TR/css3-multicol/#column4. The spec says nothing about
the z-order of column-rules relative to other rendered items. It needs to,
in terms of http://www.w3.org/TR/CSS21/zindex.html.

For a columns element that is also a stacking context, Webkit apparently
paints them in step 4 or later, e.g. above negative-zindex children of the
columns element. My preference is for the rules to be painted as part of the
background of the columns element, just above the actual background ---
i.e., for a columns element that is also a stacking context, at the end of
step 2. This seems a bit more logical to me. Anyway, the spec needs
clarification one way or another.

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]