[css-images-3] Simplify markup

Author: fantasai

css-images-3/Overview.bs

@@ -1162,7 +1162,7 @@ Object-Sizing Terminology {#sizing-terms}
 	to make it easier to refer to various concepts:
 
 	<dl export>
-		<dt id="intrinsic-dimensions"><dfn id="natural-dimensions" lt="natural dimension">natural dimensions</dfn>
+		<dt><dfn id="natural-dimensions" oldids="intrinsic-dimensions" lt="natural dimension">natural dimensions</dfn>
 		<dd>
 			The term [=natural dimensions=] refers to the set of
 			the <dfn oldids="intrinsic-height">natural height</dfn>,