[css-scoping][selectors] Per WG resolution, define that tree-abiding pseudos are allowed after ::slotted(). Fixes #1747.

Author: tabatkins

css-scoping/Overview.bs

@@ -25,6 +25,9 @@ spec:html; type:element; text:style
 spec:selectors-4; type:dfn;
 	text: static profile
 	text: dynamic profile
+spec:css-pseudo-4; type:selector; 
+	text:::before
+	text:::after
 spec:cascade-4; type:dfn; text: inherit
 </pre>
 
@@ -341,6 +344,10 @@ Selecting Slot-Assigned Content: the ''::slotted()'' pseudo-element</h4>
 	* <a lt="find flattened slotables">assigned, after flattening,</a> to the <a>slot</a> that is ''::slotted''’s originating element
 	* <a lt="match a selector against an element">matched</a> by its <<compound-selector-list>> argument
 
+	The ''::slotted()'' pseudo-element can be followed by a <a>tree-abiding pseudo-element</a>,
+	like ''::slotted()::before'',
+	representing the appropriate pseudo-element of the elements represented by the ''::slotted()'' pseudo-element.
+
 	<div class="example">
 		For example, say you had a component with both children and a shadow tree,
 		like the following:

selectors/Overview.bs

@@ -3207,16 +3207,29 @@ Grammar</h2>
 		with only a single ":" character at their start.
 	* In <<id-selector>>, the <<hash-token>>’s value must be an <a>identifier</a>.
 
-	Issue: Should I consider the "only one pseudo-element per complex selector, at the end of the last compound selector" rule now trashed?
-	In practice, all implementations violate that rule to at least some extent.
-
 	Note: The grammar above states that a combinator is optional
 	between two <<compound-selector>>s in a <<complex-selector>>.
 	This is only for grammatical purposes,
 	as the <a>CSS Value Definition Syntax</a>'s lax treatment of whitespace
 	makes it difficult to indicate that a grammar term can <em>be</em> whitespace.
 	"Omitting" a combinator is actually just specifying the <a>descendant combinator</a>.
 
+	Note: While, per the grammar, 
+	<a>pseudo-elements</a> can appear anywhere in a <<compound-selector>>,
+	and a <<compound-selector>> with a <<pseudo-element-selector>> 
+	can appear anywhere in a <<complex-selector>>,
+	in practice the <<pseudo-element-selector>> will only match anything
+	if placed at the end of the last <<compound-selector>>
+	in a <<complex-selector>>.
+	In some circumstances,
+	it can be followed by more <<pseudo-element-selector>>s or <<pseudo-class-selector>>s
+	in the same <<compount-selector>>,
+	but these are specified on a case-by-case basis.
+	(For example, the <a>user-action pseudo-classes</a> are allowed after any <a>pseudo-element</a>,
+	and the <a>tree-abiding pseudo-elements</a>
+	are allowed after the ''::slotted()'' pseudo-element)
+
+
 
 <h2 id='api-hooks'>
 API Hooks</h2>