Re: no longer treating explicit undefined as a missing, optional argument

Anne van Kesteren:
> Where are [TreatUndefinedAs=Null] and [TreatUndefined=EmptyString] used?

I don't recall if they are being used currently.  If they aren't, we 
should remove them.

> Also, how do I keep the IDL readable? I guess I could define a special
> type.
>
> typedef LegacyString = [TreatUndefinedAs=Missing] DOMString?;
>
> Though that is still not really nice.

Banging these things into the IDL rather than being in the prose often 
comes at the price of being less readable.  I don't think

   void open(DOMString method, DOMString url, optional boolean async,
             [TreatUndefinedAs=Missing] optional DOMString? user,
             [TreatUndefinedAs=Missing] optional DOMString? password);

is completely unreadable.  You may like to format the extended 
attributes in your IDL fragments differently, so that they stand out 
less.  (I've never been a fan of the "only black text on silver 
background" IDL fragment formatting in specs.)