Re: [heycam/webidl] Should probably spec a hasInstance for DOM interface objects (#129)

Can you clarify the intent of the existing [[HasInstance]]? I'm unable to understand in what ways it is different from OrdinaryHasInstance.

Is the test "is a platform object that implements the interface for which O is the interface prototype object" meant to work cross-realm? It seems like youre revised version might be or might not be, depending on whether brands are cross-realm? If that's the desired semantics, how many browsers implement that?

Web IDL's current algorithm performs an observable Get of the `"prototype"` property, as does OrdinaryHasInstance. Both also do observable proto-walks. Your algorithm omits these. Is changing that a good idea?

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