[css-properties-values-api] Added issue about pseudo attribute on ElementProxy.

Author: shans

css-properties-values-api/Overview.bs

@@ -274,6 +274,8 @@ the following attributes:
 ::  The pseudo name of the PseudoElement that this ElementProxy proxies, or
     null if this ElementProxy proxies an Element.
 
+Issue(73): Do we need the pseudo attribute on ElementProxy for level 1?
+
 The {{ApplyDescriptor}} dictionary {#the-applydescriptor-dictionary}
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