[css-display] Define interleaving of inline and block formatting contexts. Fixes #1553.

Author: tabatkins

css-display-3/Overview.bs

@@ -872,15 +872,17 @@ Appendix A: Glossary</h2>
 			effectively replaces the "anonymous inline element" concept
 			introduced in <a href="https://www.w3.org/TR/CSS2/visuren.html#anonymous">CSS2&sect;9.2.2.1</a>.</span>
 
-			A block container that contains only block-level boxes
-			establishes a new <a>block formatting context</a>
+			A block container 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: A block container box can both establish a block formatting context
+			<em>and</em> an inline formatting context simultaneously.
+
 		<dt><dfn>block box</dfn>
 		<dd>
 			A <a>block-level box</a> that is also a <a>block container</a>.
@@ -975,6 +977,12 @@ Appendix A: Glossary</h2>
 			the layout of its descendants is (generally)
 			not affected by the the rules and contents of
 			the formatting context outside the box, and vice versa.
+			However, certain types of formatting context interleave and co-exist:
+			for example, an inline formatting context exists within
+			and interacts with the block formatting context
+			of the element that establishes it,
+			and a ruby container overlays a ruby formatting context
+			over the inline formatting context in which its ruby base container participates.
 
 			<p class="example">
 				For example, in a <a>block formatting context</a>,
@@ -998,6 +1006,11 @@ Appendix A: Glossary</h2>
 		<dd>
 			Block and inline formatting contexts are defined in
 			<a href="https://www.w3.org/TR/CSS2/visuren.html#normal-flow">CSS 2.1 Section 9.4</a>.
+			Inline formatting contexts exist within
+			(are part of their containing)
+			block formatting contexts;
+			for example, line boxes belonging to the inline formatting context
+			interact with floats belonging to the block formatting context.
 
 		<dt><dfn>block formatting context root</dfn>
 		<dd>