[css-view-transitions-1][css-view-transitions-2] Initial draft for cross-document view transitions (#9022)

* WIP css-view-transitions-2

* Add examples

* Update css-view-transitions-2/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

* Update css-view-transitions-2/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

* Update css-view-transitions-2/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

* Many PR changes

* Add Vlad

* Make it explicit when we capture the old state

* Fix biblio ref

* Move HTML monkey patch

* Clarify HTML entry point in old document

* Bring back render-blocked check

* Update css-view-transitions-1/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

* Update css-view-transitions-1/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

* Update css-view-transitions-2/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

* Update css-view-transitions-2/Overview.bs

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

* Fix many PR comments

* Call ready callback on skip

* Few nits

* Fix calling the process old state captured callback

* Unintended edit

* Clear ViewTransition on old doc for BFCache

---------

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

Author: noamr

css-view-transitions-1/Overview.bs

@@ -1048,8 +1048,19 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 			Note: This is used to detect changes in the [=snapshot containing block size=],
 			which causes the transition to [=skip the view transition|skip=].
 			[Discussion of this behavior](https://github.com/w3c/csswg-drafts/issues/8045).
+
+		: <dfn>old state captured steps</dfn>
+		:: An algorithm accepting nothing, or null.
+			Initially null.
+
+			Note: this is used for cross-document view transitions.
 	</dl>
 
+	A {{ViewTransition}} must never have both an [=ViewTransition/update callback=] and a [=ViewTransition/old state captured steps=].
+
+	Note: [=ViewTransition/update callback=] is optionally set for same-document view transitions,
+		and [=ViewTransition/old state captured steps=] is set for cross-document view transitions.
+
 	The {{ViewTransition/finished}} [=getter steps=] are to return [=this's=] [=ViewTransition/finished promise=].
 
 	The {{ViewTransition/ready}} [=getter steps=] are to return [=this's=] [=ViewTransition/ready promise=].
@@ -1207,6 +1218,8 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 			If failure is returned, then [=skip the view transition=] for |transition| with an "{{InvalidStateError}}" {{DOMException}} in |transition|'s [=relevant Realm=],
 			and return.
 
+		1. If |transition|'s [=ViewTransition/process old state captured=] is not null, then call [=ViewTransition/process old state captured=] and return.
+
 		1. Set |document|'s [=document/rendering suppression for view transitions=] to true.
 
 		1. [=Queue a global task=] on the [=DOM manipulation task source=],
@@ -1587,6 +1600,8 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 			Note: Since the rejection of |transition|'s [=ViewTransition/update callback done promise=] isn't explicitly handled here,
 			if |transition|'s [=ViewTransition/update callback done promise=] rejects,
 			then |transition|'s [=ViewTransition/finished promise=] will reject with the same reason.
+
+		1. If |transition|'s [=ViewTransition/process old state captured=] is not null, then call |transition|'s [=ViewTransition/process old state captured=].
 	</div>
 
 ## [=Capture the image=] ## {#capture-the-image-algorithm}