[css-view-transitions-1] Export an algorithm to skip a transition in a hidden document (w3c#9949)

* [css-view-transitions-1] Export an algorithm to skip a transition in a hidden document

See w3c#9543

* Remove cruft

* Add assert

Author: noamr

css-view-transitions-1/Overview.bs

@@ -1662,6 +1662,19 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 			then |transition|'s [=ViewTransition/finished promise=] will reject with the same reason.
 	</div>
 
+## View transition page-visibility change steps ## {#page-visibility-change-steps}
+	<div algorithm>
+	The <dfn export>view transition page-visibility change steps</dfn> given {{Document}} |document| are:
+
+	1. If |document|'s [=Document/visibility state=] is "<code>hidden</code>", then:
+
+		1. If |document|'s [=active view transition=] is not null, then [=skip the view transition|skip=] |document|'s [=active view transition=].
+
+	1. Otherwise, [=assert=]: [=active view transition=] is null.
+
+	Note: this is called from the HTML spec.
+	</div>
+
 ## [=Capture the image=] ## {#capture-the-image-algorithm}
 
 	<div algorithm>
@@ -1939,6 +1952,7 @@ Changes from <a href="https://www.w3.org/TR/2023/WD-css-view-transitions-1-20230
 * Add animation-delay inherit to UA stylesheet rules for (::view-transition) -image-pair, -old, and -new. See <a href="https://github.com/w3c/csswg-drafts/issues/9817">issue 9817</a>.
 * Auto-skip animation when document is hidden. See <a href="https://github.com/w3c/csswg-drafts/issues/9543">issue 9543</a>.
 * Remove references to cross-document view-transitions, to keep the L1 spec clean. See <a href="https://github.com/w3c/csswg-drafts/issues/9886">Issue 9886</a>.
+* Export an algorithm to skip the active transition when the page is hidden. See <a href="https://github.com/w3c/csswg-drafts/issues/9543">issue 9543</a>.
 
 <h3 id="changes-since-2022-05-25">
 Changes from <a href="https://www.w3.org/TR/2023/WD-css-view-transitions-1-20230525/">2022-05-25 Working Draft</a>