[selectors-4] Tweak pseudo-elements intro text, restore some background info on the one-colon exception (lost in <https://hg.csswg.org/drafts/diff/8e32bf889326/selectors/Overview.bs>), revert normative requirement on authors to use two-colon syntax and mark it as an issue for discussion (since it was, afaict, never discussed, just rolled into an editorial checkin).

Author: fantasai

selectors/Overview.bs

@@ -741,12 +741,11 @@ Pseudo-classes</h3>
 
 <h3 id="pseudo-elements">Pseudo-elements</h3>
 
-	Similar to how <a>pseudo-classes</a> represent additional information
-	not directly present in the document tree
-	(or that is inconvenient to use),
-	a <dfn export>pseudo-element</dfn> is an <em>element</em>
+	Similar to how certain <a>pseudo-classes</a> represent additional state information
 	not directly present in the document tree,
-	used to create abstractions about the document tree
+	a <dfn export>pseudo-element</dfn> represents an <em>element</em>
+	not directly present in the document tree.
+	They are used to create abstractions about the document tree
 	beyond those specified by the document language.
 
 	<div class="example">
@@ -767,11 +766,21 @@ Pseudo-classes</h3>
 	<a>Pseudo-element</a> names are <a>ASCII case-insensitive</a>.
 	No <a>white space</a> is allowed between the two colons, or between the colons and the name.
 
-	Due to legacy constraints,
-	user agents must allow ''::before'', ''::after'', ''::first-line'', and ''::first-letter'' pseudo-elements
-	to be written with a single colon rather than two,
-	similar to <a>pseudo-class</a> syntax.
-	Authors must always use the double-colon syntax for these <a>pseudo-elements</a>.
+	Because <a href="http://www.w3.org/TR/CSS1">CSS Level 1</a> and <a href="http://www.w3.org/TR/CSS2">CSS Level 2</a>
+	conflated pseudo-elements and pseudo-classes by sharing a single-colon syntax for both,
+	user agents must also accept the previous one-colon notation
+	for the Level 1 &amp; 2 pseudo-elements
+	(''::before'', ''::after'', ''::first-line'', and ''::first-letter'').
+	This compatibility notation is not allowed any other <a>pseudo-elements</a>.
+
+	Issue:
+	<a href="https://hg.csswg.org/drafts/diff/8e32bf889326/selectors/Overview.bs">Tab proposes</a>
+	deprecating the old syntax and RECOMMENDing that authors not use it.
+
+	<!--
+	However, as this syntax is deprecated,
+	authors should use the Level 3+ double-colon syntax for these <a>pseudo-elements</a>.
+	-->
 
 	<a>Pseudo-elements</a> do not exist independently;
 	they belong to another element on the page,