[css-sizing] Add definitions for inner and outer size.

Author: fantasai

css-sizing/Overview.bs

@@ -72,6 +72,13 @@ Terminology</h2>
 			a <a>block size</a> and/or <a>inline size</a>;
 			alternatively a width and/or height.
 
+		<dt><dfn>inner size</dfn>
+		<dd>
+			The <a href="http://www.w3.org/TR/css2/box.html#box-dimensions">content-box</a> size of a <a>box</a>.
+		<dt><dfn>outer size</dfn>
+		<dd>
+			The <a href="http://www.w3.org/TR/css2/box.html#box-dimensions">margin-box</a> size of a <a>box</a>.
+
 		<dt><dfn id="definite" lt="definite|definite size">definite size</dfn>
 		<dd>
 			A size that can be determined without measuring content;