[css-view-transitions-2] Clarify a few nesting details

- view-transition-group is tree-scoped
- nearest/custom-ident act like contain

Closes w3c#10780
Closes w3c#10633

Author: noamr

css-view-transitions-2/Overview.bs

@@ -886,20 +886,20 @@ and by applying ''view-transition-group'' to the internal element referencing th
 
 	When the [=computed value=] of 'view-transition-name' for an element is ''view-transition-name/none'', its 'view-transition-group' [=used value=] is always resolved to ''view-transition-name/none'' as well.
 
+	The <dfn>relevant tree</dfn> for the 'view-transition-group' property is the [=tree=] that includes the element in which the property itself is declared.
+
 	<dl dfn-type=value dfn-for=view-transition-group>
 		: <dfn>normal</dfn>
-		:: The [=used value=] is the element's [=nearest containing group name=].
-
-		: <dfn>nearest</dfn>
-		:: The [=used value=] is the 'view-transition-name' [=computed value=] of the nearest ancestor whose 'view-transition-name' [=computed value=] is not ''view-transition-name/none''.
-
 		: <dfn>contain</dfn>
-		:: The [=used value=] is the element's [=nearest containing group name=].
+		:: The [=used value=] is the element's [=nearest containing group name=] given the [=relevant tree=].
+
+			Note: An element with ''view-transition-group/contain'' becomes the [=nearest containing group name=] for its [=tree/descendants=].
 
-			Note: Descendant elements are contained within this group by default, as it would be their element's [=nearest containing group name=].
+		: <dfn>nearest</dfn>
+		:: The [=used value=] is the 'view-transition-name' [=computed value=] of the nearest [=tree/ancestor=] in the [=relevant tree=] whose 'view-transition-name' [=computed value=] is not ''view-transition-name/none''.
 
 		: <dfn><<custom-ident>></dfn>
-		:: The [=used value=] is the given <<custom-ident>> if the element has an ancestor whose 'view-transition-name' [=computed value=] is that <<custom-ident>>, otherwise the element's [=nearest containing group name=]
+		:: The [=used value=] is the given <<custom-ident>> if the element has an [=tree/ancestor=] in the [=relevant tree=] whose 'view-transition-name' [=computed value=] is that <<custom-ident>>, otherwise the element's [=nearest containing group name=] given the [=relevant tree=].
 	</dl>
 
 # Algorithms # {#algorithms}
@@ -1180,9 +1180,9 @@ When capturing the old or new state for an element, perform the following steps
 ### Compute the nearest containing 'view-transition-group' ### {#vt-group-nearest-contain}
 <div algorithm="nearest containing group name">
 
-To get the <dfn>nearest containing group name</dfn> for an {{Element}} |element|, perform the following steps:
+To get the <dfn>nearest containing group name</dfn> for an {{Element}} |element| and a [=tree=] |tree|, perform the following steps:
 
-	1. Let |nearestAncestorWithContain| be |element|'s nearest ancestor whose 'view-transition-name' [=computed value=] is not ''view-transition-name/none'' and whose 'view-transition-group' [=computed value=] is ''view-transition-group/contain''.
+	1. Let |nearestAncestorWithContain| be |element|'s nearest [=tree/ancestor=] in |tree|, whose 'view-transition-name' [=computed value=] is not ''view-transition-name/none'' and whose 'view-transition-group' [=computed value=] is not ''view-transition-group/normal''.
 	1. If |nearestAncestorWithContain| is null, return ''view-transition-group/none''.
 	1. Otherwise, return |nearestAncestorWithContain|'s 'view-transition-name' [=computed value=].
 </div>