[css-view-transitions-2] Allow auto as a keyword

`auto` resolves to either the element's ID, or to a unique identifier
that is stable for the duration of the transition and is not web-observable.

Resolution: w3c#8320 (comment)
Closes w3c#8320

Author: noamr

css-view-transitions-2/Overview.bs

@@ -755,7 +755,20 @@ div.box {
 
 # Nested view-transitions # {#nested-view-transitions}
 
-## Overview ## {#nested-overview}
+## Overview ## {#auto-name-overview}
+
+## Additions to 'view-transition-name'
+
+In addition to the existing values, the 'view-transition-name' also accepts an <dfn for=view-transition-name type=value>auto</dfn> keyword.
+To resolve the [=used value=] of 'view-transition-name' for |element|:
+	1. Let |computed| be the [=computed value=] of 'view-transition-name'.
+	1. If |computed| is <css>none</css>, return null.
+	1. If |computed| is a <<custom-ident>>, return |computed|.
+	1. Assert: |computed| is <css>auto</css>.
+	1. If |element| has an associated [=Element/id=], then return |element|'s [=Element/id=].
+	1. Return a unique string, which is not a <<custom-ident>>. This string should remain consistent at least for the lifetime of |element|'s [=node document=]'s [=active view transition=].
+
+		Note: this string is not web-observable, and is used for addressing the element in internal algorithms.
 
 *This section is non-normative.*
 
@@ -1170,6 +1183,8 @@ When [[css-view-transitions-1#style-transition-pseudo-elements-algorithm|updatin
 		1. [=Multiply=] |transform| by the inverse matrix of |groupContainerElement|'s [=transform from snapshot containing block=].
 </div>
 
+
+
 <h2 id="priv" class="no-num">Privacy Considerations</h2>
 
 This specification introduces no new privacy considerations.