[css-cascade-6] Add implicit scopes (#8463)

* [css-cascade-6] Add implicit scopes

Resolves #6606.

* Allow lower bounds when root is implicit

---------

Co-authored-by: Miriam Suzanne <miriam@oddbird.net>

Author: andruud

css-cascade-6/Overview.bs

@@ -20,6 +20,7 @@ Informative Classes: ex
 
 <pre class=link-defaults>
 spec:dom; type:dfn; text:shadow tree
+spec:dom; type:dfn; for:tree; text:root
 spec:css-color-4; type:property; text:color
 spec:css-values-3; type: value; text:ex
 spec:css-conditional-3; type:at-rule; text:@media
@@ -233,27 +234,26 @@ Scoped Styles</h2>
 
 	A <dfn>scope</dfn> is a subtree or fragment of a document,
 	which can be used by selectors for more targeted matching.
-	[=Scopes=] are described in CSS through a combination of two selector lists:
-
-	* The <dfn><<scope-start>></dfn> is a <<forgiving-selector-list>>.
-		Each element matched by <<scope-start>> is a [=scoping element=],
-		creating a scope with itself as the [=scoping root=].
-	* The <dfn><<scope-end>></dfn> is a <<forgiving-selector-list>>
-		that is [=scoped selector|scoped=] by the <<scope-start>> selector,
-		with the [=scoping roots=] as [=:scope elements=].
-		Each element matched by <<scope-end>> is a [=scoping limit=].
-		The <dfn>scoping limit</dfn> elements define the lower bounds of a scope,
-		so that [=scoped selectors=] are not able to match
-		any [=scoping limit=] elements,
-		or any elements nested within them.
-
-	Each resulting [=scope=] includes a [=scoping root=] and all its descendants,
-	up to any [=scoping limit=] elements. [=Scoping limit=] elements and
-	their descendants are not included in the [=scope=].
 
-	[=Pseudo-elements=] cannot be [=scoping roots=] or [=scoping limits=];
-	they are invalid both within <<scope-start>> and <<scope-end>>.
-  
+	A [=scope=] can be formed by determining:
+
+	* The [=scoping root=] [=node=],
+		which acts as the upper bound of the scope,
+		and optionally:
+	* The <dfn>scoping limit</dfn> elements,
+		which act as the lower bounds.
+
+	An element is <dfn>in scope</dfn> if:
+
+	* It is an [=inclusive descendant=] of the [=scoping root=], and
+	* It is not an [=inclusive descendant=] of a [=scoping limit=].
+
+	Note: Only <em>elements</em> can be [=in scope=],
+		however the '':scope'' pseudo-class can match non-elements
+		when it is not the [=subject=] of a selector.
+
+	[=Scopes=] are described in CSS using the ''@scope'' rule.
+
 	Note: In contrast to [[CSS-SCOPING-1#shadow-dom|Shadow Encapsulation]],
 	which describes a persistent one-to-one relationship in the DOM
 	between a [=shadow host=] and its nested [=shadow tree=],
@@ -351,17 +351,49 @@ Scoping Styles: the ''@scope'' rule</h4>
 
 	The syntax of the ''@scope'' rule is:
 
-	<pre class='prod'>
-	@scope (<<scope-start>>) [to (<<scope-end>>)]? {
+	<pre class="prod def">
+	@scope [(<<scope-start>>)]? [to (<<scope-end>>)]? {
 	  <<stylesheet>>
 	}
 	</pre>
 
+	where:
+
+	<pre class="prod def">
+	<dfn><<scope-start>></dfn> = <<forgiving-selector-list>>
+	<dfn><<scope-end>></dfn> = <<forgiving-selector-list>>
+	</pre>
+
+	A ''@scope'' rule which specifies a <<scope-start>>
+	produces a number of <dfn>explicit scopes</dfn> as follows:
+
+	* For each element matched by <<scope-start>>,
+		create a [=scope=] using that element as the [=scoping root=].
+	* For each [=scope=] created by the above:
+		* Collect all elements that are [=in scope=]
+			and that match <<scope-end>>,
+			using the [=scoping root=] as the '':scope'' element, then
+		* Set those elements as the [=scoping limits=].
+
+	[=Pseudo-elements=] cannot be [=scoping roots=] or [=scoping limits=];
+	they are invalid both within <<scope-start>> and <<scope-end>>.
+  
+	A ''@scope'' rule which does not specify a <<scope-start>>
+	produces a single <dfn>implicit scope</dfn> as follows:
+
+	* The [=scoping root=] is the [=parent element=] of the [=owner node=]
+		of the stylesheet where the ''@scope'' rule is defined.
+		If no such element exist,
+		then the [=scoping root=] is the [=root=] of the containing [=node tree=].
+	* The [=scoping limits=] are unset;
+		[=implicit scopes=] do not have lower bounds.
+
 	The ''@scope'' [=at-rule=] has three primary effects
 	on the [=style rules=] in its <<stylesheet>>:
 
-	* [=scoped selectors|Selectors are scoped=] to the given [=scope=],
-		with the [=:scope element=] being the [=scoping root=].
+	* Selectors can only match elements that are [=in scope=].
+		This only applies to the [=subject=];
+		the rest of the selector can match unrestricted.
 	* Selectors are given the added specificity of
 		the most specific [=complex selector=] in the <<scope-start>> argument.
 
@@ -562,6 +594,9 @@ Changes since the 21 December 2021 First Public Working Draft</h3>
 	Significant changes since the
 	<a href="https://www.w3.org/TR/2021/WD-css-cascade-6-20211221/">21 December 2021 First Public Working Draft</a> include:
 
+	* Added [=implicit scopes=].
+		(<a href="https://github.com/w3c/csswg-drafts/issues/6606">Issue 6606</a>)
+
 	* Disallowed [=pseudo-elements=] in the ''@scope'' prelude.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7382">Issue 7382</a>)