[css-2018] Make bikeshed happy

Author: frivoal

css-2018/Overview.bs

@@ -127,7 +127,7 @@ Background: The W3C Process and CSS</h3>
 <h2 id="css">Cascading Style Sheets (CSS) &mdash; The Official Definition</h2>
 
 	As of 2018,
-	<dfn>Cascading Style Sheets (CSS)</dfn> is defined by the following
+	<dfn export>Cascading Style Sheets (CSS)</dfn> is defined by the following
 	specifications.
 
 	<dl>
@@ -386,7 +386,7 @@ Background: The W3C Process and CSS</h3>
 CSS Levels</h3>
 
 	Cascading Style Sheets does not have versions in the traditional sense;
-	instead it has <dfn>levels</dfn>. Each level of CSS builds on the previous,
+	instead it has <dfn export>levels</dfn>. Each level of CSS builds on the previous,
 	refining definitions and adding features. The feature set of each higher
 	level is a superset of any lower level, and the behavior allowed for a given
 	feature in a higher level is a subset of that allowed in the lower levels.