[css-cascade-6] Disallow pseudo-elements in @scope prelude (w3c#8461)

* [css-cascade-6] Disallow pseudo-elements in @scope prelude

Resolves w3c#7382.

* Spaces and tabs

---------

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

Author: andruud

css-cascade-6/Overview.bs

@@ -476,6 +476,9 @@ Scoping Styles: the ''@scope'' rule</h4>
 	are [=scoped selectors|scoped by=]
 	the selectors of the outer one.
 
+	[=Pseudo-elements=] cannot be [=scoping roots=] or [=scoping limits=];
+	they are invalid both within <<scope-start>> and <<scope-end>>.
+
 <h4 id="scope-combinator">
 Scoped Descendant Combinator</h4>
 
@@ -553,6 +556,9 @@ 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:
 
+	* Disallowed [=pseudo-elements=] in the ''@scope'' prelude.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/7382">Issue 7382</a>)
+
 	* Removed selector scoping notation.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7709">Issue 7709</a>)