[css-variables][meta] export dfn

Author: tabatkins

css-variables-1/Overview.bs

@@ -559,7 +559,7 @@ Invalid Variables</h3>
 	makes the declaration <a>invalid at computed-value time</a>,
 	unless a valid fallback is specified.
 
-	A declaration can be <dfn>invalid at computed-value time</dfn>
+	A declaration can be <dfn export>invalid at computed-value time</dfn>
 	if it contains a ''var()'' that references a <a>custom property</a> with its initial value,
 	as explained above,
 	or if it uses a valid <a>custom property</a>,