[css-inline-3][editorial] Give invisible line boxes their own section

Author: fantasai

css-inline-3/Overview.bs

@@ -180,18 +180,6 @@ Layout of Line Boxes</h3>
 	The first/last line boxes in a [=block container=] may additionally
 	be trimmed by 'leading-trim'.
 
-	[=Line boxes=] that contain no text,
-	no [=preserved white space=],
-	no [=inline boxes=] with non-zero [=margins=], [=padding=], or [=borders=],
-	and no other [=in-flow=] content
-	(such as [=atomic inlines=] or [=ruby annotations=]),
-	and do not end with a [=forced line break=]
-	must be treated as zero-[=logical height|height=] [=line boxes=]
-	for the purposes of determining the positions of any elements inside of them
-	(such as [=absolutely positioned boxes=]),
-	and must be treated as not existing for any other purpose
-	(such as <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapsing margins</a>).
-
 	<figure>
 		<img src="images/box-model.png"
 		     alt="diagram showing inline boxes split across line boxes as described above">
@@ -254,6 +242,22 @@ Layout Within Line Boxes</h3>
 	[=margins=], [=padding=], [=borders=], and a 'line-height',
 	and thus influence these calculations just like boxes with content.
 
+<h3 id="invisible-line-boxes">
+“Invisible” Line Boxes</h3>
+
+	[=Line boxes=] that contain no text,
+	no [=preserved white space=],
+	no [=inline boxes=] with non-zero [=margins=], [=padding=], or [=borders=],
+	and no other [=in-flow=] content
+	(such as [=atomic inlines=] or [=ruby annotations=]),
+	and do not end with a [=forced line break=]
+	are <dfn lt="invisible line box">invisible line boxes</dfn>.
+	Such boxes must be treated as zero-[=logical height|height=] [=line boxes=]
+	for the purposes of determining the positions of any elements inside of them
+	(such as [=absolutely positioned boxes=]),
+	and must be treated as not existing for any other purpose
+	(such as <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapsing margins</a>).
+
 <h3 id="paint-order">
 Painting Order</h3>