[css-2018] Export the 'vendor prefix' term. Fixes #3427.

Author: tabatkins

css-2018/Overview.bs

@@ -535,7 +535,7 @@ Proprietary and Non-standardized Features</h4>
 
 	To avoid clashes with future CSS features,
 	the CSS2.1 specification reserves a
-	<dfn lt="vendor prefix | vendor-prefixed | prefixed | unprefixed"><a href="https://www.w3.org/TR/CSS2/syndata.html#vendor-keywords">prefixed syntax</a></dfn> [[!CSS2]]
+	<dfn export lt="vendor prefix | vendor-prefixed | prefixed | unprefixed"><a href="https://www.w3.org/TR/CSS2/syndata.html#vendor-keywords">prefixed syntax</a></dfn> [[!CSS2]]
 	for proprietary and experimental extensions to CSS.
 	A CSS feature is a <dfn>proprietary extension</dfn> if it is meant for use
 	in a closed environment accessible only to a single vendor's user agent(s).