[css-variables] Update changes section.

Author: tabatkins

css-variables-1/Overview.bs

@@ -716,9 +716,18 @@ Serializing Custom Properties</h3>
 	property names are restricted to the ASCII range and are <a>ASCII case-insensitive</a>,
 	so implementations typically serialize the name lowercased.
 
+<h2 id='changes'>
+Changes</h2>
 
+<h3 id='changes-20151203'>
+Changes Since the 03 December 2015 CR</h3>
 
-<h2 id='changes'>
+	* Now that [[css-syntax-3]] auto-trims whitespace from declaration values,
+		made <<declaration-value>> optional in the custom property grammar,
+		so that empty variables are still allowed.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/774#issuecomment-434571254">Issue 774</a>)
+
+<h3 id='changes-20140506'>
 Changes since the May 6 2014 Last Call Working Draft</h2>
 
 	* Serialization of longhands when shorthand uses a variable was defined.