[css-display] Define block formatting context root, plus a few other minor fixes from #1471.

Author: fantasai

css-display/Overview.bs

@@ -1005,19 +1005,25 @@ Appendix A: Glossary</h2>
 			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>.
 
+		<dt><dfn>block formatting context root</dfn>
+		<dd>
+			A <a>block container</a>
+			that establishes a new <a>block formatting context</a>.
+
 		<dt><dfn>BFC</dfn>
 		<dd>
-			Abbreviation for <a>block formatting context</a>.
+			Abbreviation for <a>block formatting context</a> or
+			<a>block formatting context root</a>.
 			Has various informal definitions referring to boxes which
 			contain internal floats, exclude external floats, and suppress margin collapsing,
-			and may refer specifically to:
+			and may therefore refer specifically to one of:
 
 				* a <a>block container</a>
 					that establishes a new <a>block formatting context</a> for its contents
 				* a <a>block box</a> (i.e. a <a>block-level</a> block container)
 					that establishes a block formatting context for its contents
 					(as distinguished from a block box which does not)
-				* any block-level box that establishes a new formatting context
+				* (very loosely) any block-level box that establishes a new formatting context
 					(other than an inline formatting context)
 
 		<dt><dfn>out-of-flow</dfn>