[css-sizing] Fix issue about definite sizes that are computations against other definite sizes.

Author: fantasai

css-sizing/Overview.bs

@@ -82,9 +82,9 @@ Terminology</h2>
 		<dt><dfn id="definite" lt="definite|definite size">definite size</dfn>
 		<dd>
 			A size that can be determined without measuring content;
-			that is, a <var>&lt;length></var>,
+			that is, a <<length>>,
 			a size of the <a>initial containing block</a>,
-			or a <var>&lt;percentage></var> that is resolved against a <a>definite</a> size.
+		  or a <<percentage>> or other formula 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.