[css-sizing] Add an example of the 'other formula'. Fix markup error.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -84,7 +84,9 @@ Terminology</h2>
 			A size that can be determined without measuring content;
 			that is, a <<length>>,
 			a size of the <a>initial containing block</a>,
-		  or a <<percentage>> or other formula that is resolved solely against <a>definite</a> sizes.
+			or a <<percentage>> or other formula
+			(such the <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">“fill-available” sizing of non-replaced blocks</a> [[CSS21]])
+			that is resolved solely against <a>definite</a> sizes.
 			Additionally, the size of the <a>containing block</a> of an absolutely positioned element is always <a>definite</a>
 			with respect to that element.
 
@@ -213,7 +215,7 @@ Intrinsic Size Contributions</h3>
 	</dl>
 
 	Intrinsic size contributions are based on the <a>outer size</a> of the box;
-	for this purpose ''auto'' margins are treated as zero.
+	for this purpose ''margin/auto'' margins are treated as zero.
 
 <h3 id='constraints'>
 Intrinsic Size Constraints</h3>

css-sizing/Overview.bs

@@ -84,7 +84,9 @@ Terminology</h2>
 			A size that can be determined without measuring content;
 			that is, a <<length>>,
 			a size of the <a>initial containing block</a>,
-		  or a <<percentage>> or other formula that is resolved solely against <a>definite</a> sizes.
+			or a <<percentage>> or other formula
+			(such the <a href="https://www.w3.org/TR/CSS21/visudet.html#blockwidth">“fill-available” sizing of non-replaced blocks</a> [[CSS21]])
+			that is resolved solely against <a>definite</a> sizes.
 			Additionally, the size of the <a>containing block</a> of an absolutely positioned element is always <a>definite</a>
 			with respect to that element.
 
@@ -213,7 +215,7 @@ Intrinsic Size Contributions</h3>
 	</dl>
 
 	Intrinsic size contributions are based on the <a>outer size</a> of the box;
-	for this purpose ''auto'' margins are treated as zero.
+	for this purpose ''margin/auto'' margins are treated as zero.
 
 <h3 id='constraints'>
 Intrinsic Size Constraints</h3>