[css-align] Switch term to 'content-based', because it includes some things that are technically 'definite', like float widths. Fixes w3c#509.

Author: tabatkins

css-align-3/Overview.bs

@@ -1639,7 +1639,7 @@ Row and Column Gutters: the 'row-gap' and 'column-gap' properties</h3>
 
 			Negative values are invalid.
 			Percentages resolve to zero
-			when specified against an <a>indefinite size</a>
+			when specified against a content-based size
 			(such as the <a>logical width</a> of a float
 			or the ''height/auto'' <a>logical height</a>
 			of a <a>block-level</a> <a>grid container</a>).