[css-anchor-position] Explicitly specify that fallback styles are applied as part of a new origin, to make it clearer they win over element-attached styles. w3c#9149

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1367,14 +1367,17 @@ These modified styles are applied to the element via [=interleaving=],
 so they affect [=computed values=]
 (and can trigger transitions/etc)
 even tho they depend on layout and [=used values=].
-They are applied as part of the [=author origin=],
-as part of a theoretical UA-controlled final cascade layer.
+
+They are applied as <dfn export>position fallback origin</dfn>,
+a new [=cascade origin=] positioned between the [=author origin=]
+and the [=animation origin=].
 
 <div algorithm>
 	To <dfn>apply a position option</dfn> to an element |el|,
 	given a [=position option=] |new styles|:
 
-	1. With |new styles| inserted into the cascade at the very end of the [=author origin=],
+	1. With |new styles| inserted into the cascade
+		in the [=position fallback origin=],
 		resolve the cascade,
 		and perform enough layout to determine |el|'s [=used value|used styles=].