[css-position-3] Define the initial fixed containing block, and its relationship with the ICB. #11602

Author: tabatkins

css-position-3/Overview.bs

@@ -289,7 +289,8 @@ Containing Blocks of Positioned Boxes</h3>
 			'transform', 'will-change', 'contain'&hellip;
 
 			If no ancestor establishes one,
-			the [=fixed positioning containing block=] is:
+			the box's [=fixed positioning containing block=]
+			is the <dfn export>initial fixed containing block</dfn>:
 
 			* in [=continuous media=],
 				the [=layout viewport=]
@@ -311,6 +312,10 @@ Containing Blocks of Positioned Boxes</h3>
 			that extend outside the [=layout viewport=]/[=page area=]
 			cannot be scrolled to
 			and will not print.
+
+			The [=initial fixed containing block=]
+			is the parent of the [=initial containing block=]
+			in the [=containing block chain=].
 	</dl>
 
 <div class="example">