[css-scoping] First pass at adding :has-slotted (#10959)

* First pass at adding `:has-slotted`

`assignedNodes` reference doesn’t work

* Fix assignedNodes

* Update css-scoping-1/Overview.bs

Co-authored-by: Tab Atkins Jr. <[email protected]>

* Fix

---------

Co-authored-by: Tab Atkins Jr. <[email protected]>

css-scoping-1/Overview.bs

@@ -24,7 +24,8 @@ spec:dom; type:dfn;
 	text:find slottables
 	text:find flattened slottables
 	text:element; for:/
-spec:html; type:element; text:style
+spec:html; type:element;
+	text:style
 spec:selectors-4; type:dfn;
 	text: static profile
 	text: dynamic profile
@@ -40,7 +41,7 @@ spec:dom; type:dfn; text:child
 spec:css-color-5; type:function; text:color()
 </pre>
 
-<!-- 
+<!--
 
 <h2 id="intro">
 Introduction</h2>
@@ -507,6 +508,25 @@ Selecting Slot-Assigned Content: the ''::slotted()'' pseudo-element</h4>
 	The only way to style assigned text nodes
 	is by styling the <a>slot</a> and relying on inheritance.
 
+<h4 id='the-has-slotted-pseudo'>
+Matching on the Presence of Slot-Assigned Nodes: the '':has-slotted'' pseudo-class</h4>
+
+	The <dfn selector>:has-slotted</dfn> pseudo-class
+	matches <{slot}> elements
+	which have a non-empty list of <a lt="find slottables">slotted nodes</a>.
+
+	When '':has-slotted'' matches a slot with fallback content,
+	we can conclude that the fallback content is <em>not</em> being displayed.
+
+	Note: Even a single whitespace text node is sufficient to make '':has-slotted''' apply.
+	This is by design, so that the behavior of this pseudo-class is consistent with the behavior of the {{HTMLSlotElement/assignedNodes()}} method.
+	A future version of this specification is expected to introduce a way to exclude this case from matching.
+
+	Note: It is expected that a future version of this specification will introduce a functional '':has-slotted()'' pseudo-class that allows
+	more fine-grained matching by accepting a selector argument.
+	'':has-slotted'' is <em>not</em> an alias of '':has-slotted(*)'',
+	as the latter would not match slotted text nodes, but '':has-slotted'' does.
+
 <wpt>
 	css-scoping-shadow-slotted-nested.html
 	css-scoping-shadow-slotted-rule.html
@@ -968,4 +988,4 @@ which protect them from the things defined in this specification).
 
 <wpt hidden>
 	chrome-1492368-crash.html
-</wpt>
+</wpt>