[css-values-4] Add sub-headings to change section

Author: fantasai

css-values-4/Overview.bs

@@ -4512,6 +4512,9 @@ Acknowledgments</h2>
 <h2 class="no-num" id="changes">
 Changes</h2>
 
+<h3 class=no-num id="changes-recent">
+Recent Changes</h3>
+
 	Changes since the <a href="https://www.w3.org/TR/2018/WD-css-values-4-20181010/">10 October 2018 Working Draft</a> consist of
 	synchronizing with the <a href="https://www.w3.org/TR/2019/CR-css-values-3-20190131/#changes">recent changes in CSS Level 3</a>.
 
@@ -4521,7 +4524,10 @@ Changes</h2>
 		<li>Added rules for interpolation per value type, and clarified computed values.
 	</ul>
 
-	Changes since <a href="http://www.w3.org/TR/css-values-3/">Level 3</a>:
+<h3 class=no-num id="additions-L3">
+Additions Since Level 3</h3>
+
+	Changes since <a href="http://www.w3.org/TR/css-values-3/">CSS Values and Units Level 3</a>:
 
 	<ul>
 		<li>Added the ''vi'', ''vb'', ''ic'', ''cap'', ''lh'' and ''rlh'' units.