Merge pull request w3c#7515 from vmpstr/contentvisibilityevent

[css-contain-2] Add ContentVisibilityAutoStateChanged event (closes w3c#7384)

Author: vmpstr

css-contain-2/Overview.bs

@@ -1552,6 +1552,52 @@ Using ''content-visibility: auto''</h3>
 		will be forced to rely on the ''200px'' height estimate.
 	</div>
 
+Detecting ''content-visibility: auto'' state changes: the 'ContentVisibilityAutoStateChanged' event {#content-visibility-auto-state-changed}
+--------------------------------------------------------------------------------------------------------------------------------------------
+
+<pre class='idl'>
+[Exposed=Window]
+interface ContentVisibilityAutoStateChangedEvent : Event {
+  constructor(DOMString type, optional ContentVisibilityAutoStateChangedEventInit eventInitDict = {});
+  readonly attribute boolean skipped;
+};
+dictionary ContentVisibilityAutoStateChangedEventInit : EventInit {
+  boolean skipped = false;
+};
+</pre>
+
+<div class="attributes">
+
+Description of ContentVisibilityAutoStateChangedEvent attributes:
+
+: <dfn attribute for=ContentVisibilityAutoStateChangedEvent>skipped</dfn>
+:: Set to true if target changed state to [=skip its contents=], and false otherwise.
+
+</div>
+
+<div class="members">
+
+Description of ContentVisibilityAutoStateChangedEventInit members:
+
+: <dfn dict-member for=ContentVisibilityAutoStateChangedEventInit>skipped</dfn>
+:: See the description of the {{ContentVisibilityAutoStateChangedEvent/skipped}} attribute.
+
+</div>
+
+This event is fired on an element with ''content-visibility: auto'' style when
+the rendering state changes and the element either becomes or stops being
+[=relevant to the user=].
+
+This event is dispatched by posting a task at the time when the state change occurs.
+
+Note that elements in ''content-visibility: auto'' subtrees remain semantically
+relevant even for elements that [=skip its contents=]. This means that it is
+inappropriate to use this signal to indefinitely skip DOM updates in the
+subtree that is skipped. Instead, it should be used to deprioritize updates,
+but ensure that the content remains semantically relevant and reasonably
+up-to-date. This is particularly important for assistive technologies which
+consume this content even when the ancestor is set to [=skip its contents=].
+
 Restrictions and Clarifications {#cv-notes}
 -------------------------------------------