[selectors] Define pseudo-compounds, and that they are only present in complex selectors.

Author: tabatkins

selectors-4/Overview.bs

@@ -539,6 +539,31 @@ Structure and Terminology</h3>
 	no whitespace is allowed between the <a>simple selectors</a>
 	in a <a>compound selector</a>.
 
+	A <dfn id=pseudo-compound>pseudo-compound selector</dfn>
+	is a [=pseudo-element=] selector,
+	optionally followed by additional [=pseudo-class=] selectors,
+	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=],
+	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=].
+	</div>
+
 	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.
@@ -551,7 +576,7 @@ 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>
+	a sequence of one or more <a>compound selectors</a> and/or [=pseudo-compound selectors=]
 	separated by <a>combinators</a>.
 	It represents a set of simultaneous conditions
 	on a set of elements in the particular relationships
@@ -589,8 +614,6 @@ Structure and Terminology</h3>
 	when it matches any (at least one) of the <a>selectors</a>
 	in that <a>selector list</a>.
 
-	Issue: Pseudo-elements aren't handled here, and should be.
-
 <h3 id='data-model'>
 Data Model</h3>