add definition of "definite size"

Author: alexmog1

css3-flexbox/Overview.src.html

@@ -1127,6 +1127,11 @@ <h2 id='layout-algorithm'>
 			</dl>
 		</li>
 	</ol>
+
+	<p>For the purposes of this algorithm, measurements are considered <dfn>definite</dfn> if their precise value is known before the start of the algorithm. 
+	For example, sizes in pixels or inches are always definite; 'auto' and percent may or may not be definite, 
+	which depends on how these values were handled before this algorithm is 
+	applied. </p>
 
 <h2 id="pagination">
 Page breaks in flexbox</h2>