[css-position-3] Dfn "absolute positioning containing block" and "fixed positioning containing block", and genericize the definitions slightly to make it clearer how other specs should hook it.

Author: tabatkins

css-position-3/Overview.bs

@@ -310,17 +310,10 @@ Containing Blocks of Positioned Boxes</h3>
 	it is defined as follows:
 
 	<dl class=switch>
-		<dt id="fixed-cb">If the box has ''position: fixed'':
-		<dd>
-			The [=containing block=] is established by
-			the [=viewport=] in the case of [=continuous media=],
-			or the [=page area=] in the case of [=paged media=].
-			See [[#fixpos-insets]].
-
 		<dt id="absolute-cb">If the box has ''position: absolute'':
 		<dd>
 			The [=containing block=] is established
-			by the nearest ancestor box with a 'position' other than ''position/static'',
+			by the nearest ancestor box that establishes an <dfn export>absolute positioning containing block</dfn>,
 			in the following way:
 
 			<dl class=switch>
@@ -338,10 +331,29 @@ Containing Blocks of Positioned Boxes</h3>
 
 					Note: If the ancestor breaks across a line,
 					the “start” position might more end-ward than the “end” position.
-
-				<dt>If there is no such ancestor,
-				<dd>the [=containing block=] is the [=initial containing block=].
 			</dl>
+
+			If no ancestor establishes one,
+			the [=absolute positioning containing block=] is 
+			the [=initial containing block=].
+
+			Note: The 'position' property causes an element to establish an [=absolute positioning containing block=]
+			for any value other than ''static'';
+			other properties, such as 'contain',
+			can also do so.
+
+		<dt id="fixed-cb">If the box has ''position: fixed'':
+		<dd>
+			The [=containing block=] is established
+			by the nearest ancestor box that establishes an <dfn export>fixed positioning containing block</dfn>,
+			with the bounds of the [=containing block=]
+			determined identically to the [=absolute positioning containing block=].
+
+			If no ancestor establishes one,
+			the [=fixed positioning containing block=] is
+			the [=viewport=] in the case of [=continuous media=],
+			or the [=page area=] in the case of [=paged media=].
+			See [[#fixpos-insets]].
 	</dl>
 
 <div class="example">