[css-display-3] Merge in prose from CSS2 9.1.2 Containing Blocks into definition of containing block

Author: fantasai

css-display-3/Overview.bs

@@ -1037,16 +1037,27 @@ Appendix A: Glossary</h2>
 		<dt><dfn>containing block</dfn>
 		<dd>
 			A rectangle that forms the basis of sizing and positioning
-			for the boxes associated with it
-			(usually the children of the box that generated it).
-			Notably, a containing block is <em>not a box</em>
+			for the [=boxes=] associated with it.
+			Notably, a [=containing block=] is <em>not a [=box=]</em>
 			(it is a rectangle),
-			however it is often derived from the dimensions of a box.
-			If properties of a containing block are referenced,
-			they reference the values on the box that generated the containing block.
+			however it is often derived from the dimensions of a [=box=].
+			Each [=box=] is given a position with respect to its [=containing block=],
+			but it is not confined by this [=containing block=];
+			it can [=overflow=].
+			The phrase “a box’s containing block” means
+			“the [=containing block=] in which the box lives,”
+			not the one it generates.
+
+			In general, the [=edges=] of a [=box=]
+			act as the [=containing=] block for descendant boxes;
+			we say that a box “establishes”
+			the [=containing block=] for its descendants.
+			If properties of a [=containing block=] are referenced,
+			they reference the values on the [=box=] that generated the [=containing block=].
 			(For the <a>initial containing block</a>,
 			values are taken from the root element
 			unless otherwise specified.)
+
 			See [[CSS2]] <a href="https://www.w3.org/TR/CSS2/visuren.html#containing-block">Section 9.1.2</a>
 			and <a href="https://www.w3.org/TR/CSS2/visudet.html#containing-block-details">Section 10.1</a> for details.