[css-scoping] Define the context of a selector/declaration more rigorously, and refer to shadow-including tree order from DOM.

Author: tabatkins

css-scoping/Overview.bs

@@ -297,9 +297,10 @@ Shadow DOM Selection Model</h3>
 	and their descendants,
 	ordered by a pre-order traversal.
 
-	A selector is <dfn>in the context of a shadow tree</dfn> if,
-	when <a>match a selector against a tree</a> is called for it,
-	one of its <var>root elements</var> are in a <a>shadow tree</a>.
+	When a selector is <a lt="match a selector against a tree">matched against a tree</a>,
+	its <dfn export>tree context</dfn> is the <a>root</a> of the <var>root elements</var> passed to the algorithm.
+	If the <a>tree context</a> is a <a>shadow root</a>,
+	that selector is being matched <dfn>in the context of a shadow tree</dfn>.
 
 	<div class="example">
 		For example,
@@ -308,6 +309,9 @@ Shadow DOM Selection Model</h3>
 		So is the argument to {{querySelector()}} when called from a <a>shadow root</a>.
 	</div>
 
+	Declarations inherit the <a>tree context</a>
+	of the selector that was matched to apply them.
+
 <h4 id='host-element-in-tree'>
 Shadow Hosts in a Shadow Tree</h4>
 
@@ -619,9 +623,9 @@ Cascading</h4>
 	called Shadow Tree.
 
 	* When comparing two declarations
-		that are in different <a>shadow trees</a>,
-		then for normal rules the declaration earlier in the <a>tree-of-trees order</a> wins,
-		and for important rules the declaration coming later in the <a>tree-of-trees order</a> wins.
+		that have different <a>tree contexts</a>,
+		then for normal rules the declaration earlier in the <a>shadow-including tree order</a> wins,
+		and for important rules the declaration coming later in the <a>shadow-including tree order</a> wins.
 
 		Note: This is the <em>opposite</em> of how scoped styles work.