[css-cascade-6] Clarify @scope effects on :scope and & w3c#8377

Author: fantasai

css-cascade-6/Overview.bs

@@ -328,6 +328,13 @@ Effects of ''@scope''</h4>
 	* The [=style rules=] in an ''@scope'' <<stylesheet>>
 		are [=scoped style rules=].
 
+	* The '':scope'' selector is defined to match
+		the ''@scope'' rule’s [=scoping root=].
+		The ''&'' selector is defined to represent
+		the selector representing the [=scoping root=]
+		(the [=scope-start selector=]),
+		or else '':scope'' if no selector was specified.
+
 	* The [=cascade=] prioritizes declarations
 		with a [=scope proximity|more proximate=] [=scoping root=],
 		regardless of specificity or source order
@@ -381,6 +388,17 @@ Scoped Style Rules</h4>
 		(This only applies to the [=subject=];
 		the rest of the selector can match unrestricted.)
 
+	* They accept a <<relative-selector-list>> as their prelude
+		(rather than just a <<selector-list>>).
+		Such [=relative selectors=]
+		are relative to '':scope''.
+
+	* Any selector in the <<relative-selector-list>>
+		that does not start with a [=combinator=]
+		but does [=contain the nesting selector=] or the '':scope'' selector,
+		is interpreted as a non-[=relative selector=]
+		(but the [=subject=] must still be [=in scope=] to match).
+
 <h4 id="scope-limits">
 Identifying Scoping Roots and Limits</h4>
 
@@ -394,12 +412,15 @@ Identifying Scoping Roots and Limits</h4>
 		of the stylesheet where the ''@scope'' rule is defined.
 		(If no such element exists,
 		then the [=scoping root=] is the [=root=] of the containing [=node tree=].)
+		Any '':scope'' or ''&'' selectors in <<scope-start>>
+		are interpreted as defined for its outer context.
 
 	: Finding any [=scoping limits=]
 	:: For each [=scope=] created by a [=scoping root=],
 		its [=scoping limits=] are set to all elements
 		that are [=in scope=] and that match <<scope-end>>,
-		using the [=scoping root=] as the '':scope'' element.
+		interpreting '':scope'' and ''&''
+		exactly as in [=scoped style rules=].
 
 	<div class=example>
 		The following selectors have the same specificity (0,0,1):