[css-scoping] :host-context() ancestor path should follow distribution.

Author: tabatkins

css-scoping/Overview.bs

@@ -339,10 +339,22 @@ Selecting Into the Light: the '':host'', '':host()'', and '':host-context()'' ps
 	the '':host-context()'' pseudo-class matches the <a>host element</a>,
 	if the <a>host element</a> or one of its ancestors matches the provided <<compound-selector>>.
 	For the purpose of this pseudo-class,
-	the "ancestor" of an element is its parent,
-	if it has one,
-	or else its <a>host element</a>,
-	if it has one.
+	the "ancestor" of an element is:
+
+	<dl>
+		<dt>if the element is distributed to a <a>distribution list</a>
+		<dd>
+			the <a element>content</a> element it is ultimately distributed to.
+
+		<dt>if the element is a top-most element in a shadow tree
+		<dd>
+			the <a>host element</a>
+
+		<dt>otherwise
+		<dd>
+			the element's parent,
+			if it has one.
+	</dl>
 
 	Note: This means that the selector pierces through shadow boundaries on the way up,
 	looking for elements that match its argument,

css-scoping/Overview.html

@@ -465,10 +465,22 @@ <h4 class="heading settled heading" data-level=3.2.1 id=host-selector><span clas
 	the <a class=css data-link-type=maybe href=#selectordef-host-context title=:host-context()>:host-context()</a> pseudo-class matches the <a data-link-type=dfn href=#host-element0 title="host element">host element</a>,
 	if the <a data-link-type=dfn href=#host-element0 title="host element">host element</a> or one of its ancestors matches the provided <a class="production css-code" data-link-type=type href=http://dev.w3.org/csswg/selectors-4/#ltcompound-selector title="<compound-selector>">&lt;compound-selector&gt;</a>.
 	For the purpose of this pseudo-class,
-	the "ancestor" of an element is its parent,
-	if it has one,
-	or else its <a data-link-type=dfn href=#host-element0 title="host element">host element</a>,
-	if it has one.
+	the "ancestor" of an element is:
+
+	<dl>
+		<dt>if the element is distributed to a <a data-link-type=dfn href=#distribution-list title="distribution list">distribution list</a>
+		<dd>
+			the <a data-link-type=element title=content>content</a> element it is ultimately distributed to.
+
+		<dt>if the element is a top-most element in a shadow tree
+		<dd>
+			the <a data-link-type=dfn href=#host-element0 title="host element">host element</a>
+
+		<dt>otherwise
+		<dd>
+			the element’s parent,
+			if it has one.
+	</dl>
 
 <p class=note>	Note: This means that the selector pierces through shadow boundaries on the way up,
 	looking for elements that match its argument,