[css-values] Export 'CSS-wide keywords'.

Author: tabatkins

css-values/Overview.bs

@@ -365,7 +365,7 @@ Pre-defined Keywords</h3>
 CSS-wide keywords: ''initial'' and ''inherit''</h4>
 
 	As defined <a href="#component-types">above</a>,
-	all properties accept the <dfn>CSS-wide keywords</dfn>,
+	all properties accept the <dfn export>CSS-wide keywords</dfn>,
 	which represent value computations common to all CSS properties.
 
 	The ''initial'' keyword represents the value specified as the property's initial value.