[selectors-4] Define sub-pseudo-element terminology.

Author: fantasai

selectors-4/Overview.bs

@@ -887,6 +887,31 @@ Pseudo-classing Pseudo-elements</h4>
 	are not valid when compounded to a pseudo-element;
 	so, for example, ''::before:first-child'' is an invalid selector.
 
+<h4 id="sub-pseudo-elements">
+Sub-pseudo-elements</h4>
+
+	Some [=pseudo-elements=] are able to be the [=originating element=]
+	of other [=pseudo-elements=],
+	which are defined as the <dfn export lt="sub-pseudo-element">sub-pseudo-elements</dfn>
+	of this <dfn export>originating pseudo-element</dfn>.
+	For example,
+	when ''::before'' is given a ''list-item'' [=display type=],
+	it becomes the [=originating pseudo-element=]
+	of its ''::before::marker'' [=sub-pseudo-element=].
+
+	Where disambiguation is needed,
+	the term <dfn export>ultimate originating element</dfn>
+	refers to the real (non-pseudo) element
+	from which a [=pseudo-element=] originates.
+
+	Unless the corresponding [=sub-pseudo-element=]
+	is explicitly defined to exist in another specification,
+	pseudo-element selectors are not valid
+	when compounded to another pseudo-element selector.
+	So, for example, ''::before::before'' is an invalid selector,
+	but ''::before::marker'' is valid
+	(in implementations that support the ''::before::marker'' [=sub-pseudo-element=]).
+
 <h4 id="pseudo-element-structure">
 Internal Structure</h4>
 
@@ -3976,6 +4001,7 @@ Changes since the 21 November 2018 Working Draft</h3>
 	<ul>
 		<li>Removed the Selector profiles, marked '':has()'' as optional and at-risk instead.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/3925">Issue 3925</a>)
+		<li>Added [[#sub-pseudo-elements]] to define [=sub-pseudo-elements=] and related terminology.
 	</ul>
 
 <h3 id="changes-2018-02">