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> Element types have specific semantics that don't very across instances. I > would have thought that this would be one of the few universally accepted > truths in the XML world (wishful thinking, I guess). I think this depends on your notion of type. If I have an element containing other elements in a specific order, I can give that combination a type name "foo". If I have an instance that includes all of those elements, plus an addition element, I can say that the instance extends the type of the element, and I could give that derived type a name "bar" saying that "bar" extends "foo" (type derivation by extension in XML Schema), or I could simply say that the type of the element is "element content" for a less restrictive type. I think William was just pointing out that ad-hoc type extension is, in fact, very common in XML applications, and is one of the reasons for the success of XML.
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