https://github.com/w3c/csswg-drafts/issues/925

Copy some text from CSS 3 Color to clarify the stacking context behavior, and de-duplicate the text to avoid having this long-winded explanation in two places.

Author: Simon Fraser

css-transforms/Overview.bs

@@ -220,7 +220,9 @@ Note that an identical rendering can be obtained by nesting elements with the eq
 
 For elements whose layout is governed by the CSS box model, the transform property does not affect the flow of the content surrounding the transformed element. However, the extent of the overflow area takes into account transformed elements. This behavior is similar to what happens when elements are offset via relative positioning. Therefore, if the value of the 'overflow' property is ''overflow/scroll'' or ''overflow/auto'', scrollbars will appear as needed to see content that is transformed outside the visible area.
 
-For elements whose layout is governed by the CSS box model, any value other than ''transform/none'' for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.
+For elements whose layout is governed by the CSS box model, any value other than ''transform/none'' for the transform results in the creation of a stacking context. 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’. If an element with a transform 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
+
+For elements whose layout is governed by the CSS box model, any value other than ''transform/none'' for the transform also causes the element to become a containing block, and the object acts as a containing block for fixed positioned descendants. 
 
 Issue: Is this effect on ''position: fixed'' necessary? If so, need to go into more detail here about why fixed positioned objects should do this, i.e., that it's much harder to implement otherwise. See <a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=16328">Bug 16328</a>.
 
@@ -246,7 +248,7 @@ Media: visual
 Animatable: as <a href="#interpolation-of-transforms">transform</a>
 </pre>
 
-Any computed value other than ''transform/none'' for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.
+Any computed value other than ''transform/none'' for the transform affects containing block and stacking context, as described in [[#transform-rendering]].
 
 <pre class=prod><dfn>&lt;transform-list></dfn> = <<transform-function>>+</pre>