[selectors] Move the legacy pseudo-element syntax into the grammar itself, for clarity. No normative change intended. #8122

Author: tabatkins

selectors-4/Overview.bs

@@ -3543,7 +3543,9 @@ Grammar</h2>
 	<dfn>&lt;pseudo-class-selector></dfn> = ':' <<ident-token>> |
 	                          ':' <<function-token>> <<any-value>> ')'
 
-	<dfn>&lt;pseudo-element-selector></dfn> = ':' <<pseudo-class-selector>>
+	<dfn>&lt;pseudo-element-selector></dfn> = ':' <<pseudo-class-selector>> | <<legacy-pseudo-element-selector>>
+
+	<dfn>&lt;legacy-pseudo-element-selector></dfn> =  ':' [before | after | first-line | first-letter]
 	</pre>
 
 	In interpreting the above grammar,
@@ -3568,14 +3570,14 @@ Grammar</h2>
 		between two <<compound-selector>>s
 		if the <<combinator>> between them is omitted.
 		(This indicates the descendant combinator is being used.)
-	<li id="single-colon-pseudos">
-		The four <a href="https://www.w3.org/TR/CSS2/selectors.html#pseudo-element-selectors">Level 2</a> <a>pseudo-elements</a>
-		(''::before'', ''::after'', ''::first-line'', and ''::first-letter'')
-		may, for legacy reasons,
-		be represented using the <<pseudo-class-selector>> grammar,
-		with only a single ":" character at their start.
 	<li>
 		In <<id-selector>>, the <<hash-token>>’s value must be an <a>identifier</a>.
+	<li>
+		The <<pseudo-class-selector>> production
+		excludes the <<legacy-pseudo-element-selector>> production.
+		(That is, '':before''/etc must never be parsed as a pseudo-class,
+		even if doing so would cause the selector to become valid
+		due to, for example, other simple selectors following it.)
 	</ul>
 
 	Note: A selector is also subject to a variety of more specific syntactic constraints,
@@ -3594,6 +3596,13 @@ Grammar</h2>
 	and the <a>tree-abiding pseudo-elements</a>
 	are allowed after the ''::slotted()'' pseudo-element.)
 
+	<p class=note id=single-colon-pseudos>
+		The four <a href="https://www.w3.org/TR/CSS2/selectors.html#pseudo-element-selectors">Level 2</a> <a>pseudo-elements</a>
+		(''::before'', ''::after'', ''::first-line'', and ''::first-letter'')
+		may, for legacy reasons,
+		be written with only a single ":" character at their front,
+		making them resemble a <<pseudo-class-selector>>.
+
 <h3 id='forgiving-selector'>
 <<forgiving-selector-list>> and <<forgiving-relative-selector-list>></h3>
 
@@ -4051,6 +4060,8 @@ Changes since the 7 May 2022 Working Draft</h3>
 		(<a href="https://github.com/w3c/csswg-drafts/issues/3760">Issue 3760</a>)
 	* Defined '':-webkit-autofill'' [=legacy selector alias=].
 		(<a href="https://github.com/w3c/csswg-drafts/issues/7474">Issue 7474</a>)
+	* Moved the legacy single-colon pseudo-element syntax into the grammar itself.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/8122">Issue 8122</a>)
 
 <h3 id="changes-2018-11">
 Changes since the 21 November 2018 Working Draft</h3>