[css-sizing-4] Define ratio-determining axis.

Author: fantasai

css-sizing-4/Overview.bs

@@ -156,9 +156,13 @@ Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>
 	and <a href="https://www.w3.org/TR/css-flexbox-1/#algo-main-item">CSS Flexible Box Model Level 1 &sect;&#x202F;9.2</a>.
 	The axis in which the <a>preferred size</a> calculation
 	depends on this aspect ratio
-	is called the <dfn>ratio-dependent axis</dfn>,
+	is called the <dfn export>ratio-dependent axis</dfn>,
 	and the resulting size is <a>definite</a>
 	if its input sizes are also <a>definite</a>.
+	The opposite axis (on which the [=ratio-dependent axis=] size depends)
+	is the <dfn export>ratio-determining axis</dfn>.
+
+
 
 	<dl dfn-for="aspect-ratio" dfn-type="value">
 		<dt><dfn>auto</dfn>