[css-flexbox] Ref Sizing for definite/indefinite instead.

Author: tabatkins

css-flexbox/Overview.bs

@@ -2679,14 +2679,8 @@ Resolving Flexible Lengths</h3>
 <h3 id='definite-sizes'>
 Definite and Indefinite Sizes</h3>
 
-	For the purposes of these definitions,
-	a <dfn lt="definite|definite size">definite size</dfn> is one that can be determined without measuring content,
-	i.e. is a <<length>>,
-	a size of the initial containing block,
-	or a <<percentage>> that is resolved against a definite size.
-	An <dfn lt="indefinite|indefinite size">indefinite size</dfn> is one that is not <a>definite</a>.
-
-	Flexbox has several additional cases where a length can be considered <a>definite</a>:
+	Although CSS Sizing [[!CSS3-SIZING]] defines <a spec=css-sizing>definite</a> and <a spec=css-sizing>indefinite</a> lengths,
+	Flexbox has several additional cases where a length can be considered <dfn noexport lt="definite|definite size|indefinite|indefinite size">definite</dfn>:
 
 	<ol>
 		<li>