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a supplemental interface that more than one object can implement. None of
the members of the Region interface can clash with any of the members of
the object that implements it.

Right now Elements can become CSS Regions, but in the future other objects
will be able to become CSS Regions. As far as I know, the correct way to
specify this kind of relation is with WebIDL supplemental interfaces. I'd
rather figure out the correct way to add this WebIDL functionality to
WebKit now, than put something else into the spec and WebKit that we'll
have to change later.

Thanks,

Alan