[css-color-4] Restore text about non-opaque elements painting at layer 0 even when not positioned. Slightly reword surrounding text to be more readable. Fixes #3793.

Author: tabatkins

css-color-4/Overview.bs

@@ -2442,7 +2442,7 @@ Transparency: the 'opacity' property</h2>
 
 	Opacity can be thought of as a postprocessing operation.
 	Conceptually, after the element (including its descendants) is rendered into an RGBA offscreen image,
-	the  opacity setting specifies how to blend the offscreen rendering into
+	the opacity setting specifies how to blend the offscreen rendering into
 	the current composite rendering.
 	See <a href="#alpha">simple alpha compositing</a> for details.
 
@@ -2473,17 +2473,22 @@ Transparency: the 'opacity' property</h2>
 	an opaque child occluding part of the element's background will continue to do so even when 'opacity' is less than 1,
 	but the element and child as a whole will show the underlying page through themselves.
 
-	If 'opacity' has a value less than 1,
-	the element forms a <a>stacking context</a>
-	for its children.
-	This means that any content outside of it cannot be layered in z-order between pieces of content inside of it,
-	and vice versa.
-	If the element is in a context where the 'z-index' property applies,
-	the ''z-index/auto'' value is treated as ''0'' for the element.
+	If a box has 'opacity' less than 1,
+	it forms a <a>stacking context</a> for its children.
+	(This prevents its contents from interleaving in the z-axis
+	with content outside it.)
+
+	Furthermore, if the 'z-index' property applies to the box,
+	the ''z-index/auto'' value is treated as ''0'' for the element;
+	it is otherwise painted on the same layer within its parent stacking context
+	as positioned elements with stack level 0
+	(as if it were a positioned element with ''z-index:0'').
+
 	See <a href="https://www.w3.org/TR/CSS21/visuren.html#layers">section 9.9</a>
 	and <a href="https://www.w3.org/TR/CSS21/zindex.html">Appendix E</a> of [[!CSS21]]
 	for more information on stacking contexts.
-	The rules in this paragraph do not apply to SVG elements,
+
+	These rules about z-order do not apply to SVG elements,
 	since SVG has its own <a href="https://www.w3.org/TR/SVG11/render.html">rendering model</a> ([[!SVG11]], Chapter 3).