[css-inline-3] Export line box

Author: fantasai

css-inline-3/Overview.bs

@@ -86,7 +86,7 @@ Inline Layout Box Model</h2>
 	(and their margins do not <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">collapse</a>).
 	The rectangular area that contains the boxes
 	that form a line of [=inline-level content=]
-	is called a <dfn>line box</dfn>.
+	is called a <dfn export>line box</dfn>.
 
 	Note: <a>Line boxes</a> and <a>inline boxes</a> and <a>inline-level boxes</a>
 	are each different things!