[css-display-3] Define root element term and display type. w3c#7786 w3c#6480 w3c#7207

Author: fantasai

css-display-3/Overview.bs

@@ -861,7 +861,13 @@ Automatic Box Type Transformations</h3>
 		</ul>
 	</div>
 
-	The root element’s display type is always <a>blockified</a>.
+<h3 id='root'>
+The Root Element’s Principal Box</h3>
+
+	The [=root element=]’s display type is always <a>blockified</a>,
+	and its [=principal box=] always establishes an [=independent formatting context=].
+	This box’s [=containing block=] is the [=initial containing block=].
+
 	Additionally, a 'display' of ''display/contents'' computes to ''block'' on the root element.
 
 <!--
@@ -1384,6 +1390,14 @@ Appendix A: Glossary</h2>
 	The following terms are defined here for convenience:
 
 	<dl export>
+		<dt><dfn>root element</dfn>
+		<dd>
+			The [=CSS/element=] at the root of the [=document tree=].
+			In a [=document tree=] produced under the DOM,
+			this is the [=document element=];
+			in HTML it is the <{html}> element.
+			[[DOM]] [[HTML]]
+
 		<dt><dfn>principal box</dfn>
 		<dd>
 			When an [=CSS/element=] generates one or more [=boxes=],
@@ -1547,9 +1561,11 @@ Appendix A: Glossary</h2>
 
 		<dt><dfn>initial containing block</dfn>
 		<dd>
-			The <a>containing block</a> of the root element.
+			The <a>containing block</a> of the [=root element=].
+			The [=initial containing block=] establishes a [=block formatting context=].
 			See <a href="https://www.w3.org/TR/CSS2/visudet.html#containing-block-details">CSS2.1&sect;10.1</a> for [=continuous media=];
-			and [[!CSS-PAGE-3]] for [=paged media=].
+			and [[!CSS-PAGE-3]] for [=paged media=]
+			for its position and dimensions.
 
 		<dt><dfn>formatting context</dfn>
 		<dd>