[css-cascade-4] Define inheritance over flattened tree for DOM with shadows.

Author: fantasai

css-cascade-4/Overview.bs

@@ -1055,7 +1055,12 @@ Inheritance</h3>
 	which has no parent element,
 	the <a>inherited value</a> is the <a>initial value</a> of the property.
 
-	(Pseudo-elements inherit according to a fictional tag sequence described for each pseudo-element [[!SELECT]].)
+	For a [[DOM]] tree with shadows,
+	inheritance operates on the [=flattened element tree=].
+	<span class="note">This means that slotted elements inherit from the <{slot}> they're assigned to,
+	rather than directly from their [=light tree=] parent.</span>
+	[=Pseudo-elements=] inherit according to the fictional tag sequence
+	described for each [=pseudo-element=]. [[!CSS-PSEUDO-4]]
 
 	Some properties are <dfn export lt="inherited property">inherited properties</dfn>,
 	as defined in their property definition table.
@@ -1149,6 +1154,9 @@ Changes Since the 28 August 2018 Candidate Recommendation</h3>
 			Added <a href="#cascade-context">context</a> to the [=cascade=] sort criteria
 			to accommodate Shadow DOM. [[DOM]]
 			(<a href="https://github.com/w3c/csswg-drafts/issues/5372">Issue 5372</a>)
+		<li id="change-2018-shadow-inherit">
+			Defined that, in consideration of [=shadow trees=],
+			[=inheritance=] operates over the [=flattened element tree=].
 	</ul>
 
 <h3 id="changes-2016">