[css-anchor-position-1] Apply only allowed properties when applying fallback style (#9044)

Co-authored-by: Tab Atkins Jr <jackalmage@gmail.com>

Author: xiaochengh

css-anchor-position-1/Overview.bs

@@ -878,7 +878,8 @@ is the rule's name.
 If multiple ''@position-fallback'' rules are declared with the same name,
 the last one in document order "wins".
 
-The ''@try'' rule only accepts the following [=properties=]:
+The ''@try'' rule only <dfn lt="accepted @try properties">accepts</dfn>
+the following [=properties=]:
 
 * [=inset properties=]
 * [=sizing properties=]
@@ -1007,7 +1008,8 @@ to trigger automatic "animation" of the fallback'd properties.
 
 	2. [=list/For each=] |fallback styles| in the [=position fallback list=]:
 
-		1. Apply the styles in |fallback styles| to |el|,
+		1. Apply the values of the [=accepted @try properties=]
+			in |fallback styles| to |el|,
 			overriding the corresponding properties in |base styles|.
 
 			Perform any specified/computed/used-value time normalizations