[css-display] Define new term 'containing block chain' because css-overflow wants to use it, and maybe some other specs too.

Author: fantasai

css-display/Overview.bs

@@ -741,6 +741,18 @@ Appendix A: Glossary</h2>
 			See [[CSS2]] <a href="http://www.w3.org/TR/CSS2/visuren.html#containing-block">Section 9.1.2</a>
 			and <a href="http://www.w3.org/TR/CSS2/visudet.html#containing-block-details">Section 10.1</a> for details.
 
+		<dt><dfn>containing block chain</dfn>
+		<dd>
+			A sequence of successive <a>containing blocks</a>
+			that form an ancestor-descendant chain
+			through the <a>containing block</a> relation.
+			For example, an <a>inline box</a>’s containing block is the content box of its closest <a>block container</a> ancestor;
+			if that block container is an in-flow <a>block</a>,
+			then <em>its</em> containing block is formed by its parent <a>block container</a>;
+			if that grandparent <a>block container</a> is <a>absolutely positioned</a>,
+			then <em>its</em> containing block is the padding edges of its closest <em>positioned</em> ancestor (not necessarily its parent),
+			and so on up to the <a>initial containing block</a>.
+
 		<dt><dfn>initial containing block</dfn>
 		<dd>
 			The <a>containing block</a> of the root element.