[css-display] Add 'principal box' and 'replaced element' to the glossary.

Author: tabatkins

css-display/Overview.bs

@@ -738,12 +738,27 @@ Run-In Layout</h2>
 
 	Note: This run-in model is slightly different from the one proposed in earlier revisions of [[!CSS2]].
 
-<h2 id='glossary'>
+<h2 id='glossary' class=no-num>
 Appendix A: Glossary</h2>
 
 	The following terms are defined here for convenience:
 
 	<dl export>
+		<dt><dfn>principal box</dfn>
+		<dd>
+			When an [=element=] generates one or more [=boxes=],
+			one of them is the [=principal box=],
+			which contains its descendant boxes and generated content,
+			and is also the box involved in any positioning scheme.
+
+			Some elements may generate additional boxes in addition to the principal box
+			(such as ''display/list-item'' elements,
+			which generate an additional marker box,
+			or ''display/table'' elements,
+			which generate a [=principal box|principal=] [=table wrapper box=]
+			and an additional [=table box=].
+			These additional boxes are placed with respect to the principal box.
+
 		<dt><dfn lt="inline-level|inline-level content">inline-level</dfn>
 		<dd>
 			Content that participates in inline layout.
@@ -781,6 +796,28 @@ Appendix A: Glossary</h2>
 			Used as a shorthand for <a>block box</a>, <a>block-level box</a>, or <a>block container box</a>,
 			where unambiguous.
 
+		<dt><dfn>replaced element</dfn>
+		<dd>
+			An element whose content is outside the scope of the CSS formatting model,
+			such as an image, embedded document, or applet.
+			<span class=non-normative>
+				For example, the content of the HTML <{img}> element
+				is often replaced by the image that its <{img/src}> attribute designates.
+			</span>
+			Replaced elements often have intrinsic dimensions:
+			an intrinsic width, an intrinsic height, and an intrinsic ratio.
+			For example, a bitmap image has an intrinsic width and an intrinsic height specified in absolute units
+			(from which the intrinsic ratio can obviously be determined).
+			On the other hand, other documents may not have any intrinsic dimensions
+			(for example, a blank HTML document).
+
+			User agents may consider a replaced element to not have any intrinsic dimensions
+			if it is believed that those dimensions could leak sensitive information to a third party.
+			For example, if an HTML document changed intrinsic size depending on the user's bank balance,
+			then the UA might want to act as if that resource had no intrinsic dimensions.
+
+			The content of replaced elements is not considered in the CSS rendering model.
+
 		<dt><dfn>containing block</dfn>
 		<dd>
 			A rectangle that forms the basis of sizing and positioning