[selectors-4][editorial] Rephrase the pseudo-compound and complex selector parts a bit. #5676

Author: tabatkins

selectors-4/Overview.bs

@@ -542,28 +542,38 @@ Structure and Terminology</h3>
 	A <dfn id=pseudo-compound>pseudo-compound selector</dfn>
 	is a [=pseudo-element=] selector,
 	optionally followed by additional [=pseudo-class=] selectors,
+	and optionally preceded by a [=compound selector=]
+	or another [=pseudo-compound selector=],
 	without any [=combinators=].
 	(A <a>pseudo-compound selector</a> is represented by <<pseudo-compound-selector>>
 	in the selectors <a href="#grammar">grammar</a>.)
 	A [=pseudo-element=] [=matches=] a [=pseudo-compound selector=]
 	when it has the specified pseudo-element name,
 	matches the additional conditions represented by any [=pseudo-classes=],
 	and has an [=originating element=]
-	represented by the adjacent preceding [=compound selector=].
-	If there is no adjacent preceding [=compound selector=],
+	represented by the adjacent preceding selector.
+	If there is no adjacent preceding selector,
 	the [=universal selector=] is assumed.
 	(For example, ''.foo ::before''
 	is equivalent to ''.foo *::before'',
 	and distinct from ''.foo::before''.)
 
 	<div class=example>
-	For example, in ''.foo::before:hover'',
-	the ''.foo'' is a [=compound selector=],
-	while the ''::before:hover'' is a [=pseudo-compound selector=].
-	However, in ''.foo::before::marker'',
-	''::before'' and ''::marker'' are separate [=pseudo-compound selectors=].
+		For example, in ''.foo::before:hover'',
+		the ''.foo'' is a [=compound selector=],
+		while the ''::before:hover'' is a [=pseudo-compound selector=].
+		However, in ''.foo::before::marker'',
+		''::before'' and ''::marker'' are separate [=pseudo-compound selectors=].
 	</div>
 
+	Note: A [=pseudo-compound selector=] <strong>is not</strong>
+	a [=compound selector=],
+	and can't be used in places that expect a [=compound selector=] only.
+	[=Pseudo-compound selectors=] act as if they carry a [=combinator=] with themselves,
+	expressing their relationship with their [=originating element=],
+	just as the ''>'' combinator
+	expresses a relationship with a parent element.
+
 	A <dfn export for=selector>combinator</dfn>
 	is a condition of relationship between two elements
 	represented by the <a>compound selectors</a> on either side.
@@ -576,27 +586,36 @@ Structure and Terminology</h3>
 	when the condition of relationship between these elements is true.
 
 	A <dfn id="complex" export>complex selector</dfn> is
-	a sequence of one or more <a>compound selectors</a> and/or [=pseudo-compound selectors=]
-	separated by <a>combinators</a>.
+	a sequence of one or more <a>compound selectors</a>
+	and/or [=pseudo-compound selectors=],
+	with [=compound selectors=] separated by <a>combinators</a>.
 	It represents a set of simultaneous conditions
 	on a set of elements in the particular relationships
 	described by its <a>combinators</a>.
 	(Complex selectors are represented by <<complex-selector>>
 	in the selectors <a href="#grammar">grammar</a>.)
-	A given element is said to <a>match</a> a <a>complex selector</a>
-	when there exists a list of elements,
-	each matching a corresponding <a>compound selector</a> in the <a>complex selector</a>,
-	with each pair of elements consecutive in the list matching
-	the <a>combinator</a> between their corresponding <a>compound selectors</a>,
-	and with the last element being the given element.
-
-	Note: Thus, a selector consisting of a single <a>compound selector</a>
-	matches any element satisfying the requirements
-	of its constituent <a>simple selectors</a>.
-	Prepending another <a>compound selector</a> and a <a>combinator</a>
-	to a sequence imposes additional matching constraints,
-	such that the <a>subjects</a> of a <a>complex selector</a> are always
-	a subset of the elements represented by its last <a>compound selector</a>.
+	A given element or pseudo-element
+	is said to <a>match</a> a <a>complex selector</a>
+	when it matches the final [=compound selector|compound=]/[=pseudo-compound selector=] in the sequence,
+	and every preceding unit of the sequence also [=matches=]
+	an element or [=pseudo-element=],
+	with the correct relationship between consecutive units
+	as expressed by the combinators separating them
+	(or, for [=pseudo-compound selectors=],
+	the correct [=originating element=] relationship).
+
+	<div class=example>
+		For example, ''.foo.bar'' matches an element
+		with both "foo" and "bar" classes.
+
+		''.ancestor > .foo.bar'' matches a subset of those elements:
+		only those whose parent element
+		(as indicated by the ''>'' combinator)
+		has the "ancestor" class.
+
+		''.foo.bar::before'' matches a ''::before'' pseudo-element,
+		whose [=originating element=] matches ''.foo.bar''.
+	</div>
 
 	A <dfn export lt="list of simple selectors|list of compound selectors|list of complex selectors">list of simple/compound/complex selectors</dfn>
 	is a comma-separated list of