[css-overflow-3] Add per-axis subtypes of scroll containers

Author: fantasai

css-overflow-3/Overview.bs

@@ -321,15 +321,34 @@ Scrolling Overflow</h3>
 -->
 
 <h2 id="overflow-properties">
-Scrolling and Clipping Overflow</h2>
+Clipping and Scrolling Overflow</h2>
+
+	The properties in this chapter specify whether and where a box’s [=overflow=] is clipped;
+	if so, whether it is a [=scroll container=];
+	and if so, in which axis(es) it is allowed to scroll,
+	thus which of the following types of [=scroll container=] it is:
+
+	<dl>
+		<dt><dfn>single-axis scroll container</dfn>
+		<dd>
+			A [=scroll container=] that can scroll in only one axis.
+
+		<dt><dfn>dual-axis scroll container</dfn>
+		<dd>
+			A [=scroll container=] that can scroll in both axes.
+
+		<dt><dfn>block-axis scroll container</dfn>
+		<dt><dfn>inline-axis scroll container</dfn>
+		<dt><dfn>x-axis scroll container</dfn>
+		<dt><dfn>y-axis scroll container</dfn>
+		<dd>
+			A [=scroll container=] that can scroll in the specified axis,
+			regardless of whether it can also scroll in the other axis.
+	</dl>
 
 <h3 id="overflow-control">
 Managing Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h3>
 
-	These properties specify whether a box’s [=overflow=] is clipped,
-	and if so,
-	whether it is a [=scroll container=].
-
 	<pre class=propdef>
 		Name: overflow-x, overflow-y, overflow-block, overflow-inline
 		Value: visible | hidden | clip | scroll | auto