[css-anchor-position] More explicitly restrict what pseudos can be an anchor. w3c#8615

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -268,6 +268,13 @@ or nothing (a missing specifier).
 		before reaching |query el|'s [=containing block=]
 		is either not [=absolutely positioned=]
 		or precedes |query el| in the tree order.
+
+	* |el| is either an [=element=],
+		or a [=pseudo-element=] that acts like one.
+
+		Issue: define the term that actually means this,
+		matching ::before/after/backdrop/etc
+		(but not ::marker/placeholder/etc whose box is not detectable).
 </div>
 
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