stop bikeshed complaining about CSS21 vs CSS2

Author: svgeesus

css-color-4/Overview.bs

@@ -2541,7 +2541,7 @@ Transparency: the 'opacity' property</h2>
 	(as if it were a positioned element with ''z-index:0'').
 
 	See <a href="https://www.w3.org/TR/CSS21/visuren.html#layers">section 9.9</a>
-	and <a href="https://www.w3.org/TR/CSS21/zindex.html">Appendix E</a> of [[!CSS21]]
+	and <a href="https://www.w3.org/TR/CSS21/zindex.html">Appendix E</a> of [[!CSS2]]
 	for more information on stacking contexts.
 
 	These rules about z-order do not apply to SVG elements,