[css-properties-values-api] Allow @Property in shadow trees (#1085)

Co-authored-by: Anders <andruud@dib.lan>

Author: andruud

css-properties-values-api/Overview.bs

@@ -584,13 +584,6 @@ A custom property registration from {{registerProperty()|CSS.registerProperty()}
 further wins over any ''@property'' rules
 for the same <<custom-property-name>>.
 
-A ''@property'' is invalid if it occurs in a stylesheet inside of a [=shadow tree=],
-and must be ignored.
-
-Issue(939): This will likely change in the future,
-as the behavior of concept-defining at-rules in shadow trees
-becomes more consistently defined.
-
 The '@property/syntax' Descriptor {#the-syntax-descriptor}
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