[css-position-3] Define stacking behavior of positioned boxes. #1053

Author: fantasai

css-position-3/Overview.bs

@@ -19,6 +19,7 @@ Link Defaults: css-transforms-1 (property) transform
 spec:css-writing-modes-4; type:dfn; text:end
 spec:css2; type:value; for:float; text:none
 spec:css-align-3; type:value; for:align-self; text:center
+spec:css2; type:dfn; text:stacking context
 </pre>
 
 <style type="text/css">
@@ -446,6 +447,25 @@ Containing Blocks of Positioned Boxes</h3>
 	(i.e., that generated by "div1").
 </div>
 
+<h3 id="stacking">
+Painting Order and Stacking Contexts</h3>
+
+	The 'z-index' property applies to all [=positioned boxes=].
+	When 'z-index' is ''z-index/auto'':
+
+	* [=fixed positioned|Fixed=] and [=sticky positioned|sticky=] [=positioned boxes=]
+		nonetheless form a [=stacking context=].
+	* [=relative positioned|Relative=] and [=absolute positioned|absolute=] [=positioned boxes=]
+		do not form a [=stacking context=],
+		but are painted as if those elements did generated new [=stacking contexts=],
+		except that their [=positioned=] descendants and any would-be child [=stacking contexts=]
+		take part in the current [=stacking context=].
+
+	See <a href="https://www.w3.org/TR/CSS2/visuren.html#layers">CSS2 &sect; 9.9 
+	Layered presentation</a>
+	and <a href="https://www.w3.org/TR/CSS2/zindex.html">Appendix E: 
+	Elaborate description of Stacking Contexts</a>
+	for details about 'z-index', [=stacking contexts=], and painting order.
 
 <h2 id="coords">
 Positioning Coordinates</h2>