[css-nesting-1] Clarify what it means to 'contain a nesting selector', and the CSSWG resolution to preserve forgiving selectors that are invalid but contain the nesting selector. #7972

Author: tabatkins

css-nesting-1/Overview.bs

@@ -34,7 +34,8 @@ Introduction</h2>
 Module Interactions</h3>
 
 	This module introduces new parser rules that extend the [[!CSS21]] parser model.
-	This module introduces selectors that extend the [[SELECTORS4]] module.
+	It introduces selectors that extend the [[SELECTORS-4]] module.
+	It extends and modifies some IDL and algorithms defined in the [[CSSOM-1]] module.
 
 <h3 id="values">
 Values</h3>
@@ -268,7 +269,7 @@ Syntax {#syntax}
 		are relative to the elements represented by the [=nesting selector=].
 	* If a selector in the <<relative-selector-list>>
 		does not start with a [=combinator=]
-		but does contain the [=nesting selector=],
+		but does [=contain the nesting selector=],
 		it is interpreted as a non-[=relative selector=].
 
 	The precise details of how nested style rules are parsed
@@ -570,6 +571,31 @@ Syntax {#syntax}
 		</pre>
 	</div>
 
+	<div algorithm>
+		A selector is said to <dfn export>contain the nesting selector</dfn>
+		if, when it was [=CSS/parsed=] as any type of selector,
+		a <<delim-token>> with the value "&" (U+0026 AMPERSAND) was encountered.
+
+		Note: This is phrased in this explicit manner
+		so as to catch cases like '':is(&:unknown, .bar)'',
+		which discards its first argument during parsing
+		(since it contains an unknown pseudo-class,
+		and thus is an [=invalid selector=]).
+		A selector like this still [=contains the nesting selector=].
+	</div>
+
+	If a <<forgiving-selector-list>> has an item that [=contains the nesting selector=]
+	but is invalid,
+	it is preserved exactly as-is
+	rather than being discarded.
+	(This does not change the matching behavior of the selector--
+	an invalid selector still fails to match anything--
+	just the serialization of the selector.)
+
+	Issue: The preceding paragraph needs to move to Selectors
+	when we move ''&'' itself to Selectors;
+	I'm monkey-patching for convenience here.
+
 
 
 <!--
@@ -1171,4 +1197,4 @@ that starts with an ident or function token,
 do nothing and return.
 
 Issue: The above paragraph will be inlined into the CSSOM algo,
-rather than monkey-patched.
+rather than monkey-patched.