Index: source
===================================================================
--- source	(revision 2090)
+++ source	(revision 2091)
@@ -5868,7 +5868,8 @@
   their elements). If both the <code title="attr-lang">lang</code>
   attribute and the <code title="attr-xml-lang">xml:lang</code>
   attribute are specified on the same element, they must have exactly
-  the same value.</p>
+  the same value when compared in an <span>ASCII
+  case-insensitive</span> manner.</p>
 
   <p>Authors must not use the <code
   title="attr-xml-lang">xml:lang</code> attribute (that is, the <code
@@ -5881,7 +5882,8 @@
   elements</span> in <span>HTML documents</span>, but such attributes
   must only be specified if a <code title="attr-lang">lang</code>
   attribute is also specified, and both attributes must have the same
-  value.</p>
+  value when compared in an <span>ASCII case-insensitive</span>
+  manner.</p>
 
   <hr>