[css-view-transitions-2] Update the algorithm to check opt-in on outbound transition

Instead of relying on the source snapshot params in HTML, we update a flag
every time something relevant changes, and HTML can read that flag.

Author: noamr

css-view-transitions-2/Overview.bs

@@ -435,6 +435,10 @@ Note: as per default behavior, the ''@view-transition'' rule can be nested insid
 	The '<dfn for="@view-transition">type</dfn>' descriptor sets the [=ViewTransition/active types=] for the transition
 	when capturing and performing the transition, equivalent to calling {{Document/startViewTransition(callbackOptions)}} with that {{StartViewTransitionOptions/type}}.
 
+## Responding to changes in the ''@view-transition'' rule ## {#respond-to-rule-changes}
+
+When the ''@view-transition'' rule changes for {{Document}} |document|, [=update the opt-in state for outbound transitions=] given |document|.
+
 # API # {#api}
 
 ## Additions to {{Document}} ## {#additions-to-document-api}
@@ -512,6 +516,12 @@ The {{CSSViewTransitionRule}} represents a ''@view-transition'' rule.
 		: <dfn>inbound view transition params</dfn>
 		:: a [=view transition params=], or null.
 			Initially null.
+
+		: <dfn export>can initiate outbound view transition</dfn>
+		:: a boolean.
+			Initially false.
+
+			Note: this value can be read [=in parallel=] while navigating.
 	</dl>
 
 ### The View transition params struct ### {#view-transition-params-struct}
@@ -706,6 +716,10 @@ Prepend this to the [=Perform pending transition operations=] algorithm given a
 
 		1. [=Assert=]: |document| is [=fully active=].
 
+		1. [=Assert=]: |document| [=has been revealed=] is true.
+
+		1. [=Update the opt-in state for outbound transitions=] for |document|.
+
 		1. Let |inboundViewTransitionParams| be |document|'s [=inbound view transition params=].
 
 		1. If |inboundViewTransitionParams| is null, then return null.
@@ -739,6 +753,14 @@ Prepend this to the [=Perform pending transition operations=] algorithm given a
 		1. Return |transition|.
 	</div>
 
+### Updating the opt-in state for outbound view transitions ### {#update-opt-in-state-for-outbound}
+
+<div algorithm="update-opt-in-for-outbound">
+To <dfn>update the opt-in state for outbound transitions</dfn> for a {{Document}} |document|:
+	1. If |document| [=has been revealed=], and the result of [=resolve @view-transition rule|resolving the @view-transition rule=] is not "<code>skip transition</code>",
+		then set |document|'s [=can initiate outbound view transition=] to true.
+	1. Otherwise, set |document|'s [=can initiate outbound view transition=] to false.
+
 
 <h2 id="priv" class="no-num">Privacy Considerations</h2>