[selectors-4] Add :popover-open pseudo-class. (#11362)

This is as resolved in #8637.

Fixes #8637.

Author: dbaron

selectors-4/Overview.bs

@@ -2632,6 +2632,23 @@ Collapse State: the '':open'' pseudo-class</h3>
 	Note: A ":closed" pseudo-class might be added in the future
 	once the full set of things that support '':open'' is known.
 
+<h3 id="popover-open-state">
+Popover State: the '':popover-open'' pseudo-class</h3>
+
+	The <dfn>:popover-open</dfn> pseudo-class represents an element
+	that has both “popover-showing” and “popover-hidden” states
+	and which is currently in the “popover-showing” state.
+
+	Note: This is distinct from '':open'' because
+	an element can have element-specific open and closed states
+	and also have separate popover-showing and popover-hidden states
+	associated with being a popover.
+
+	Exactly what “popover-showing” and “popover-hidden” states
+	mean is host-language specific,
+	but is exemplified by the presence of the HTML <{html-global/popover}> attribute
+	and the associated <a>popover visibility state</a>.
+
 <h3 id="modal-state">
 Modal (Exclusive Interaction) State: the '':modal'' pseudo-class</h3>
 
@@ -4192,8 +4209,11 @@ Changes since the 7 May 2022 Working Draft</h3>
 	Significant changes since the <a href="https://www.w3.org/TR/2022/WD-selectors-4-20220507/">7 May 2022 Working Draft</a>:
 
 	* Marked '':blank'' as at-risk and removed the at-risk status from '':read-write'' and '':has()''
-	* Added '':open'' and '':closed'' pseudo-classes.
-		(<a href="https://github.com/w3c/csswg-drafts/issues/7319">Issue 7319</a>)
+	* Added '':open'' pseudo-class.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/7319">Issue 7319</a>,
+		<a href="https://github.com/w3c/csswg-drafts/issues/11039">Issue 11039</a>)
+	* Added '':popover-open'' pseudo-class.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/8637">Issue 8637</a>)
 	* Disallowed [=pseudo-elements=] from '':has()'' unless explicitly allowed
 		by the pseudo-element’s definition.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7463">Issue 7463</a>)