[css-inline-3] Clarify implications of line box invisibility

Author: fantasai

css-inline-3/Overview.bs

@@ -253,10 +253,28 @@ Layout Within Line Boxes</h3>
 	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
+	for the purposes of determining the positions of any descendant content
 	(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>).
+	and both the [=line box=] and its [=in-flow=] content
+	must be treated as not existing for any other layout or rendering purpose.
+
+	<details class=note>
+		<summary>What’s invisible?</summary>
+
+		Such [=invisible line boxes=], which can still contain
+		unstyled empty [=inline boxes=], [=out-of-flow=] boxes, and/or collapsed [=document white space=],
+		are ignored, for example, for:
+		* <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">margin collapsing</a>
+		* finding the [=first formatted line=]
+		* applying 'leading-trim'
+		* [[css-break-4#break-propagation|fragmentation break propagation]]
+	</details>
+
+	ISSUE: Firefox allows the inline boxes within an [=invisible line box=]
+	to accept 'outline',which allows it to make focus rings visible.
+	As in other browsers, all other properties that could make the element visible
+	(e.g. 'box-shadow')
+	seem to be ignored.
 
 <h3 id="paint-order">
 Painting Order</h3>