[selectors-4] In a shadow tree, an unscoped :scope matches :host (not :root). #11000

Author: tabatkins

selectors-4/Overview.bs

@@ -28,6 +28,7 @@ At Risk: [=user action pseudo-classes=] applying to non-[=tree-abiding pseudo-el
 At Risk: the '':blank'' pseudo-class
 Ignored Terms: function token, Document, DocumentFragment, math, h1, shadow tree, querySelector(), quirks mode, button, a, span, object, p, div, q, area, link, label, input, html, em, li, ol, pre, CSS Value Definition Syntax
 Ignored Vars: identifier, i
+Ignore MDN Failure: #the-target-within-pseudo
 </pre>
 <pre class=link-defaults>
 spec:css-values-4; type:dfn; text:identifier
@@ -2287,10 +2288,11 @@ The Reference Element Pseudo-class: '':scope''</h3>
 	or a virtual one (such as a {{DocumentFragment}}).
 
 	If there is no [=scoping root=]
-	then '':scope'' represents the root of the document
-	(equivalent to '':root'').
+	then '':scope'' represents the root of the tree the element is in
+	(equivalent to '':host'' in a [=shadow tree=],
+	or '':root'' otherwise).
 	Specifications intending for this pseudo-class to match specific elements
-	rather than the document's root element
+	rather than this tree root element
 	must define their [=scoping root|scoping root(s)=].
 
 	A virtual [=scoping root=] is some object representing the root of a document fragment,
@@ -2338,9 +2340,9 @@ User Action Pseudo-classes</h2>
 	match on that element's ancestors as well:
 	'':hover'', '':active'', '':focus-within''.
 	Specifically, if these match on a given element,
-	they also match on the element's [=flat-tree=] ancestors,
+	they also match on the element's [=flat tree=] ancestors,
 	up to and including the first [=top layer=] element
-	or the [=root=] element,
+	or the [=tree/root=] element,
 	whichever is encountered first.
 
 	Note: The specifics of hit-testing,