[css-variables] Animation-taint the display property as well. Fixes #501.

Author: tabatkins

css-variables/Overview.bs

@@ -470,7 +470,11 @@ Using Cascading Variables: the ''var()'' notation</h2>
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 			If the <a>custom property</a> named by the first argument to the ''var()'' function is <a>animation-tainted</a>,
-			and the ''var()'' function is being used in the 'animation' property or one of its longhands,
+			and the ''var()'' function is being used in:
+
+			* the 'animation' property or one of its longhands,
+			* the 'display' property
+
 			treat the <a>custom property</a> as having its initial value for the rest of this algorithm.
 
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