[css-sizing-4] Clarify which elements can get a last remembered size, and when lrs is removed. #6220

Author: tabatkins

css-sizing-4/Overview.bs

@@ -540,7 +540,7 @@ Overriding Contained Intrinsic Sizes: the 'contain-intrinsic-*' properties</h3>
 		: <dfn>auto && <<length>></dfn>
 		:: The corresponding axis has an [=explicit intrinsic inner size=]
 			of its [=last remembered size=].
-			If a [=last remembered size=] does not yet exist,
+			If a [=last remembered size=] does not exist,
 			instead use the specified <<length>>.
 	</dl>
 
@@ -601,19 +601,28 @@ Last Remembered Size</h4>
 	offering the performance benefits of [=size containment=]
 	while <em>probably</em> sizing accurately to its contents.
 
+	Only elements capable of being {{ResizeObserver}} targets can have a [=last remembered size=].
+
 	<div algorithm="last remembered size">
 		The [=last remembered size=] of an element is determined by:
 
-		* At the time that ResizeObserver events are determined and delivered,
-			if an element has ''contain-intrinsic-size: auto''
+		* At the time that {{ResizeObserver}} events are determined and delivered,
+			if an element has ''contain-intrinsic-size: auto'',
+			is capable of being a {{ResizeObserver}} target,
 			but does not have [=size containment=],
-			record its current inner dimensions
+			record the current inner dimensions of its [=principal box=]
 			as its [=last remembered size=].
 
-		An element might not have a [=last remembered size=],
-		if it has never been rendered without [=size containment=].
-		(In this case, it will instead use the fallback value
-		provided along with ''contain-intrinsic-size/auto''.)
+		* At the time that {{ResizeObserver}} events are determined and delivered,
+			if an element has a [=last remembered size=]
+			but does <em>not</em> have ''contain-intrinsic-size: auto'',
+			remove its [=last remembered size=].
+
+		Note: The [=last remembered size=] is state attached to the <em>element</em>,
+		not any particular box generated by the element.
+		So long as the element retains ''contain-intrinsic-size: auto'',
+		it will remember its [=last remembered size=]
+		even across changes such as going to/from ''display: none''.
 	</div>
 
 <h4 id='cis-scrollbars'>