[css-pseudo-4] Single-colon syntax applies also to ::first-letter and ::first-line. Fixes #2150

Author: fantasai

css-pseudo-4/Overview.bs

@@ -698,11 +698,6 @@ Generated Content Pseudo-elements: ''::before'' and ''::after''</h3>
   may be included in any ''::first-line'' and ''::first-letter'' pseudo-elements
   applied to its <a>originating element</a>.
 
-  <p>For compatibility with existing style sheets written against CSS Level 2 [[CSS21]],
-  user agents must also accept the previous one-colon notation
-  (<code>:before</code> and <code>:after</code>)
-  for these pseudo-elements.
-
 <h3 id="marker-pseudo">
 List Markers: the ''::marker'' pseudo-element</h3>
 
@@ -928,6 +923,14 @@ Addition to the <code>window</code> interface</h3>
   Its return value is a <code>CSSPseudoElementList</code>,
   potentially empty if no pseudo-element exists for the given element and the given type.
 
+<h2 id="css2-compat">
+Compatibilitiy Syntax</h2>
+
+  <p>For compatibility with existing style sheets written against CSS Level 2 [[CSS21]],
+  user agents must also accept the previous one-colon notation
+  (<code>:before</code>, <code>:after</code>, <code>:first-letter</code>, <code>:first-line</code>)
+  for the ''::before'', ''::after'', ''::first-letter'', and ''::first-line'' pseudo-elements.
+
 <h2 class="no-num" id="changes">
 Changes</h2>