[css-anchor-position] Specify where fallback rules live in the cascade. Specify that bang-important is invalid. w3c#10058 w3c#9149

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1422,8 +1422,6 @@ is the rule's name.
 If multiple ''@position-try'' rules are declared with the same name,
 the last one in document order "wins".
 
-Issue: Or should they cascade together?
-
 The ''@position-try'' rule only <dfn export lt="accepted @position-try properties">accepts</dfn>
 the following [=properties=]:
 
@@ -1434,6 +1432,21 @@ the following [=properties=]:
 * 'position-anchor'
 * 'inset-area'
 
+It is invalid to use ''!important'' on the properties in the <<declaration-list>>.
+
+All of the properties in a ''@position-try'' are applied to the element
+as part of the <dfn>Position Fallback Origin</dfn>,
+a new [=cascade origin=] that lies between
+the [=Author Origin=]
+and the [=Animation Origin=].
+
+Similar to the [=Animation Origin=],
+use of the ''revert'' or ''revert-layer'' values
+(or anything else that would rollback the property to the preceding origin)
+acts as if the property was part of the [=Author Origin=].
+(Thus, it instead reverts back to the [=User Origin=].)
+
+
 Issue(#9195): What exactly are the constraints that determine what's allowed here?
 Current list is based off of what's reasonable
 from Chrome's experimental impl.