[css-variables] Define 'case sensitive'.

Author: tabatkins

css-variables/Overview.bs

@@ -99,7 +99,7 @@ Defining Custom Properties: the '--*' family of properties</h2>
 	</div>
 
 	Unlike other CSS properties,
-	custom property names are <strong>case-sensitive</strong>.
+	custom property names are <a>case-sensitive</a>.
 
 	<div class='example'>
 		While both ''--foo'' and ''--FOO'' are valid,