[css-display] More correctly define block container. Fixes #1185.

Author: tabatkins

css-display/Overview.bs

@@ -805,9 +805,31 @@ Appendix A: Glossary</h2>
 
 		<dt><dfn lt="block container|block container box">block container</dfn>
 		<dd>
-			A box whose contents participate in a <a>block formatting context</a>.
-			A block container does not necessarily establish a <em>new</em> <a>block formatting context</a>
-			if its parent formatting context is also a <a>block formatting context</a>.
+			A block container either contains only inline-level boxes
+			participating in an <a>inline formatting context</a>,
+			or contains only block-level boxes
+			participating in a <a>block formatting context</a>
+			(possibly generating anonymous block boxes to ensure this constraint,
+			as defined in <a href="https://www.w3.org/TR/CSS2/visuren.html#anonymous-block-level">CSS2.1&sect;9.2.1.1</a>).
+
+			A block container that contains only inline-level content
+			establishes a new <a>inline formatting context</a>.
+			The element then also generates a <a>root inline box</a>
+			which wraps all of its inline content.
+
+			A block container that contains only block-level boxes
+			establishes a new <a>block formatting context</a>
+			if its parent formatting context is <em>not</em> a <a>block formatting context</a>;
+			otherwise, when participating in a <a>block formatting context</a> itself,
+			it either establishes a new <a>block formatting context</a> for its contents
+			or continues the one in which it participates,
+			as determined by the constraints of other properties
+			(such as 'overflow' or 'align-content').
+
+			Note: Not all block container boxes are block-level boxes:
+			non-replaced inline blocks and non-replaced table cells are block containers
+			but not block-level boxes.
+			Block-level boxes that are also block containers are called <a>block boxes</a>.
 
 		<dt><dfn>block box</dfn>
 		<dd>