[css-anchor-position-1] Rewrite valid anchor rules into a less confusing form. w3c#11602

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -425,37 +425,24 @@ in anchor positioning.
 
 	* |possible anchor| is laid out strictly before |positioned el|,
 		aka one of the following is true:
-		* |positioned el| is [=in a higher top layer=] than |possible anchor|
-		* Both elements are [=in the same top layer=]
-			but have different [=containing blocks=], 
-			and |positioned el|'s [=containing block=]
-			is an ancestor of |possible anchor|'s [=containing block=]
-			in the [=containing block chain=],
-			aka one of the following:
-			* |positioned el|'s [=containing block=] is the viewport,
-				and |possible anchor|'s [=containing block=] isn't.
-			* |positioned el|'s [=containing block=] is the [=initial containing block=],
-				and |possible anchor|'s [=containing block=] is generated by an element,
-				and the last [=containing block=] in |possible anchor|'s [=containing block chain=]
-				before reaching the [=initial containing block=]
-				is either not [=absolutely positioned=]
-				or precedes |positioned el| in the tree order.
-			* both elements' [=containing blocks=] are generated by elements,
-				and |positioned el|'s containing block
-				is an ancestor in the [=flat tree=]
-				to that of |possible anchor|'s [=containing block=],
-				and the last [=containing block=] in |possible anchor|'s [=containing block chain=]
-				before reaching |positioned el|'s [=containing block=]
-				is either not [=absolutely positioned=]
-				or precedes |positioned el| in the tree order.
-		* Both elements are [=in the same top layer=]
-			and have the same [=containing block=],
-			and are both [=absolutely positioned=],
-			and |possible anchor| is earlier in [=flat tree=] order
-			than |positioned el|.
-		* Both elements are [=in the same top layer=]
-			and have the same [=containing block=],
-			but |possible anchor| isn't [=absolutely positioned=].
+		* Both |positioned el| and |possible anchor| are [=absolutely positioned=],
+			and |positioned el|'s [=absolute positioning containing block=] is
+			an ancestor of |possible anchor|'s [=absolute positioning containing block=]
+			in the [=containing block chain=].
+
+			Note: The viewport is the highest possible containing block,
+			and an ancestor of all other containing blocks.
+			The [=initial containing block=] is a child of the viewport,
+			and an ancestor of all [=containing blocks=]
+			generated by elements not in a [=top layer=].
+		* Both |positioned el| and |possible anchor| are [=absolutely positioned=]
+			and have the same [=absolute positioning containing block=],
+			and either |positioned el| is [=in a higher top layer=] than |possible anchor|,
+			or they're both in the same [=top layer=]
+			and |positioned el| is later in [=flat tree=] order than |possible anchor|.
+		* |possible anchor| is not [=absolutely positioned=],
+			and its [=containing block=]
+			is either the same as or a descendant of |positioned el|'s [=absolute positioning containing block=].
 
 	* If |possible anchor| is in the [=skipped contents=] of another element,
 		then |positioned el| is in the [=skipped contents=] of that same element.