[css-values] Dfn font-relative lengths and viewport-percentage lengths.

Author: tabatkins

css-values/Overview.bs

@@ -669,7 +669,7 @@ Relative lengths</h3>
 Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 
 	Aside from ''rem'' (which refers to the font-size of the root element),
-	the font-relative lengths refer to the font metrics of the element on which they are used.
+	the <dfn export>font-relative lengths</dfn> refer to the font metrics of the element on which they are used.
 	The exception is when they occur in the value of the 'font-size' property itself,
 	in which case they refer to the computed font metrics of the parent element
 	(or the computed font metrics corresponding to the initial values of the 'font' property,
@@ -726,7 +726,7 @@ Font-relative lengths: the ''em'', ''ex'', ''ch'', ''rem'' units</h4>
 <h4 id="viewport-relative-lengths">
 Viewport-percentage lengths: the ''vw'', ''vh'', ''vmin'', ''vmax'' units</h4>
 
-	The viewport-percentage lengths are relative to the size of the
+	The <dfn export>viewport-percentage lengths</dfn> are relative to the size of the
 	<a href="http://www.w3.org/TR/CSS21/visudet.html#containing-block-details">initial containing block</a>.
 	When the height or width of the initial containing block is changed,
 	they are scaled accordingly.