Clarify that observable image size refers to the border-box

Ink-overflow is an implementation detail, and up to the UA
to apply. object-view-box doesn't directly affect the ink-overflow rect.

See [resolution](w3c#8597 (comment)).

Closes w3c#8597

css-view-transitions-1/Overview.bs

@@ -1500,8 +1500,10 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 
 		1. [=Assert=]: |transition|'s [=ViewTransition/phase=] is not "`done`".
 
-		1. If |transition|'s [=ViewTransition/phase=] is [=phases/before=] "`update-callback-called`", then [=queue a global task=] on the [=DOM manipulation task source=],
-			given |transition|'s [=relevant global object=], to [=call the update callback=] of |transition|.
+		1. If |transition|'s [=ViewTransition/phase=] is [=phases/before=] "`update-callback-called`",
+			then [=queue a global task=] on the [=DOM manipulation task source=],
+			given |transition|'s [=relevant global object=],
+			to [=call the update callback=] of |transition|.
 
 		1. Set [=document/transition suppressing rendering=] to false.
 
@@ -1581,7 +1583,14 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 	<div algorithm>
 		The <dfn>capture rendering characteristics</dfn> are as follows:
 
-		* The origin of |element|'s [=ink overflow rectangle=] is anchored to canvas origin.
+		* The origin of |element|'s [=border box=] is anchored to canvas origin.
+
+		* Expand the canvas to include |element|'s [=ink overflow rectangle=]. The captured image
+		  should include the [=ink overflow region=], however the size of that region is
+		  [=implementation-defined=] and is not web-observable.
+
+		  Note: This means that ''object-view-box'' applies to |element|'s [=border box=], and UAs
+		  have to apply the [=ink overflow region=] to the canvas internally.
 
 		* If the referenced element has a transform applied to it (or its ancestors),
 			then the transform is ignored.
@@ -1607,16 +1616,16 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 		To <dfn lt="computing the interest rectangle|compute the interest rectangle">compute the interest rectangle</dfn> of an {{Element}} |element|, perform the following steps.
 		They return a rectangle.
 
-		Note: The interest rectangle is the subset of |element|'s [=ink overflow rectangle=] that should be captured.
-			This is required for cases where an element's ink overflow rectangle needs to be clipped because of hardware constraints.
+		Note: The interest rectangle is the subset of |element|'s [=border box=] that should be captured.
+			This is required for cases where an element's border box needs to be clipped because of hardware constraints.
 			For example, if it exceeds the maximum texture size.
 
 		1. Assert: |element| is not |element|'s [=node document=]'s [=document element=].
 
 			Note: The [=document element=] is captured differently, as specified in [=capture the image=].
 
-		1. If |element|'s [=ink overflow area=] does not exceed an implementation-defined maximum size,
-			then return a rectangle that is equal to |element|'s [=ink overflow rectangle=].
+		1. If |element|'s [=border area=] does not exceed an implementation-defined maximum size,
+			then return a rectangle that is equal to |element|'s [=border box=].
 
 		1. Otherwise: