[css-view-transitions-1][editorial] Improve view-transition-name definition

* Give it a proper title
* Give it a conceptual definition
* State that it *will* get snapshotted, rather than it might
* Make exclusion of 'none' from <custom-ident> normative
* Various wording tweaks

Author: fantasai

css-view-transitions-1/Overview.bs

@@ -483,7 +483,7 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 
 # CSS properties # {#css-properties}
 
-## 'view-transition-name' ## {#view-transition-name-prop}
+## Tagging Individually Transitioning Subtrees: the 'view-transition-name' property ## {#view-transition-name-prop}
 
 	<pre class=propdef>
 	Name: view-transition-name
@@ -495,27 +495,35 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 	Animation type: discrete
 	</pre>
 
-	The 'view-transition-name' property "names" an element as participating in a view transition.
+	The 'view-transition-name' property “tags” an element
+	as [=involved in a view transition=],
+	capturing it independently in the [=view transition tree=]
+	under the specified <dfn>view transition name</dfn>.
+	An element so captured is animated independently of the rest of the page.
 
 	<dl dfn-type=value dfn-for=view-transition-name>
 		: <dfn>none</dfn>
-		:: The [=/element=] will not participate in a view transition.
+		:: The [=/element=] will not participate independently in a view transition.
 
 		: <dfn><<custom-ident>></dfn>
-		:: The [=/element=] can participate in a view transition,
+		:: The [=/element=] will participate in a view transition,
 			as either an old or new [=/element=],
-			with a <dfn dfn for>view transition name</dfn> equal to the <<custom-ident>>'s value.
+			with the specified [=view transition name=].
 
-			Note: The value <css>none</css> is invalid as a <<custom-ident>>.
+			The value <css>none</css> is excluded from <<custom-ident>> here.
+
+			Note: If this name is not unique
+			(i.e. if two elements simultaneously specify the same [=view transition name=])
+			then the [=view transition=] will abort.
 	</dl>
 
-	Note: This property causes the user-agent to both capture separate snapshots from the elements,
-	as well as create separate pseudo-element sub-trees representing this element's “old” and “new” states.
-	For the purposes of this API,
-	if one element has a transition-name "foo" in the old state, and another element has a transition-name "foo" in the new state,
-	they are treated as representing different visual state of the same element.
+	Note:  For the purposes of this API,
+	if one element has [=view transition name=] ''foo'' in the old state,
+	and another element has [=view transition name=] ''foo'' in the new state,
+	they are treated as representing different visual state of the same element,
+	and will be paired in the [=view transition tree=].
 	This may be confusing, since the elements themselves are not necessarily referring to the same object,
-	but it is a useful model to consider them to be visual states of the same conceptual page entity, that we happen to call "element".
+	but it is a useful model to consider them to be visual states of the same conceptual page entity.
 
 # Layout and rendering changes # {#layout-rendering-changes}
 
@@ -602,14 +610,14 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 		<dfn>&lt;pt-name-selector></dfn> = '*' | <<custom-ident>>
 	</pre>
 
-	The selector matches if the <<pt-name-selector>> is `*` or matches the [=named view-transition pseudo-element=]'s [=named view-transition pseudo-element/view-transition name=].
+	The selector matches if the <<pt-name-selector>> is `*` or matches the [=named view-transition pseudo-element=]'s [=view transition name=].
 
 	The specificity of a view-transition selector with a <<custom-ident>> argument is the same as for other pseudo-elements,
 	and is equivalent to a [=type selector=].
 
 	The specificity of a view-transition selector with a `*` argument is zero.
 
-	Note: The [=named view-transition pseudo-element/view-transition name=] is set to the 'view-transition-name' that triggered its creation.
+	Note: The [=view transition name=] is set to the 'view-transition-name' that triggered its creation.
 
 ## View transition pseudo-elements ## {#view-transition-pseudos}
 
@@ -1352,27 +1360,27 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 		1. [=map/For each=] |transitionName| → |capturedElement| of |transition|'s [=ViewTransition/named elements=]:
 
 			1. Let |group| be a new ''::view-transition-group()'',
-				with its [=named view-transition pseudo-element/view-transition name=] set to |transitionName|.
+				with its [=view transition name=] set to |transitionName|.
 
 			1. Append |group| to |transition|'s [=ViewTransition/transition root pseudo-element=].
 
 			1. Let |imagePair| be a new ''::view-transition-image-pair()'',
-				with its [=named view-transition pseudo-element/view-transition name=] set to |transitionName|.
+				with its [=view transition name=] set to |transitionName|.
 
 			1. Append |imagePair| to |group|.
 
 			1. If |capturedElement|'s [=captured element/old image=] is not null, then:
 
 				1. Let |old| be a new ''::view-transition-old()'',
-					with its [=named view-transition pseudo-element/view-transition name=] set to |transitionName|,
+					with its [=view transition name=] set to |transitionName|,
 					displaying |capturedElement|'s [=captured element/old image=].
 
 				1. Append |old| to |imagePair|.
 
 			1. If |capturedElement|'s [=new element=] is not null, then:
 
 				1. Let |new| be a new ''::view-transition-new()'',
-					with its [=named view-transition pseudo-element/view-transition name=] set to |transitionName|.
+					with its [=view transition name=] set to |transitionName|.
 
 					Note: The styling of this pseudo is handled in [=update pseudo-element styles=].
 
@@ -1707,7 +1715,7 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 
 			1. If |capturedElement|'s [=new element=] is not null, then:
 
-				1. Let |new| be the ''::view-transition-new()'' with the [=view-transition name=] |transitionName|.
+				1. Let |new| be the ''::view-transition-new()'' with the [=view transition name=] |transitionName|.
 
 				1. Set |new|'s [=replaced element=] content to the result of [=capturing the image=] of |capturedElement|'s [=new element=].