[selectors-4] Allow logical combination pseudos wherever their arguments are themselves valid. #7085 (#8041)

Author: fantasai

selectors-4/Overview.bs

@@ -812,8 +812,6 @@ Pseudo-classing Pseudo-elements</h4>
 	unless otherwise-specified, none of these <a>pseudo-classes</a>
 	will match on the <a>pseudo-element</a>.
 
-	Issue: Clarify that '':not()'' and '':is()'' can be used when containing above-mentioned pseudos.
-
 	<div class="example">
 		For example, since the '':hover'' pseudo-class specifies
 		that it can apply to any pseudo-element,
@@ -831,8 +829,14 @@ Pseudo-classing Pseudo-elements</h4>
 		whereas ''::first-line:hover'' only matches if the first line itself is hovered.
 	</div>
 
+	The [=logical combination pseudo-classes=]
+	'':not()'', '':is()'', and '':where()''
+	are also valid immediately following a [=pseudo-element=]
+	provided their arguments all adhere to the above restriction.
+
 	Note: Note that, unless otherwise specified in a future specification,
 	pseudo-classes other than the <a href="#useraction-pseudos">user action pseudo-classes</a>
+	(and logical combination pseudo-classes restricted to them)
 	are not valid when compounded to a pseudo-element;
 	so, for example, ''::before:first-child'' is an invalid selector.
 
@@ -988,6 +992,22 @@ Invalid Selectors and Error Handling</h3>
 <h2 id="logical-combination">
 Logical Combinations</h2>
 
+	Selector logic can be manipulated by
+	[=compound selector|compounding=] (logical AND),
+	[=selector lists=] (logical OR),
+	and the <dfn>logical combination pseudo-classes</dfn>
+	'':is()'', '':where()'', and '':not()''.
+	The [=logical combination pseudo-classes=]
+	are allowed anywhere that any other [=pseudo-classes=] are allowed,
+	but pass any restrictions to their arguments.
+	(For example, if only [=compound selectors=] are allowed,
+	then only [=compound selectors=] are valid within an '':is()''.)
+
+	Note: Since inside '':is()'' and '':where()''
+	invalid arguments are dropped without invaliding the [=pseudo-class=] itself,
+	selector arguments that are invalidated by contextual restrictions
+	likewise do not invalidate the '':is()'' pseudo-class itself.
+
 <h3 id="grouping">
 Selector Lists</h3>