[css3-color] Describe the effect of opacity on z-ordering more clearly and make it clear that "treated as" does not imply a change in the computed value.

This is already reflected in
http://www.w3.org/Style/2011/REC-css3-color-20110607-errata.html
and was resolved in
http://lists.w3.org/Archives/Public/www-style/2011Jul/0094.html

Author: dbaron

css3-color/Overview.html

@@ -539,15 +539,13 @@ <h3 id=transparency><span class=secno>3.2. </span>Transparency: the ‘<a
    between pieces of content inside of it. For the same reason,
    implementations must create a new stacking context for any element with
    opacity less than 1. If an element with opacity less than 1 is not
-   positioned, implementations must paint the layer it creates, within its
-   parent stacking context, at the same stacking order that would be used if
-   it were a positioned element with ‘<code class=css>z-index: 0</code>’
-   and ‘<code class=css>opacity: 1</code>’. If an element with opacity
-   less than 1 is positioned, the ‘<code class=property>z-index</code>’
-   property applies as described in <a href="#CSS21"
-   rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, except that ‘<code
-   class=css>auto</code>’ is treated as ‘<code class=css>0</code>’
-   since a new stacking context is always created. See <a
+   positioned, then it is painted on the same layer, within its parent
+   stacking context, as positioned elements with stack level 0. If an element
+   with opacity less than 1 is positioned, the ‘<code
+   class=property>z-index</code>’ property applies as described in <a
+   href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>, except that if
+   the used value is ‘<code class=css>auto</code>’ then the element
+   behaves exactly as if it were ‘<code class=css>0</code>’. See <a
    href="http://www.w3.org/TR/CSS21/visuren.html#layers">section 9.9</a> and
    <a href="http://www.w3.org/TR/CSS21/zindex.html">Appendix E</a> of <a
    href="#CSS21" rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a> for more

css3-color/Overview.src.html

@@ -346,14 +346,13 @@ <h3 id="transparency">Transparency: the 'opacity' property</h3>
 in z-order between pieces of content inside of it.  For the same
 reason, implementations must create a new stacking context for
 any element with opacity less than 1.  If an element with
-opacity less than 1 is not positioned, implementations must
-paint the layer it creates, within its parent stacking context,
-at the same stacking order that would be used if it were a
-positioned element with ''z-index: 0'' and ''opacity: 1''.  If an
+opacity less than 1 is not positioned, then it is painted on the
+same layer, within its parent stacking context,
+as positioned elements with stack level 0.  If an
 element with opacity less than 1 is positioned, the 'z-index'
-property applies as described in [[!CSS21]], except that ''auto''
-is treated as ''0'' since a new stacking context is always
-created.  See <a
+property applies as described in [[!CSS21]], except that
+if the used value is ''auto'' then the element behaves exactly as
+if it were ''0''.  See <a
 href="http://www.w3.org/TR/CSS21/visuren.html#layers">section
 9.9</a> and <a
 href="http://www.w3.org/TR/CSS21/zindex.html">Appendix E</a> of