DTD and Schema Teasers

Two only slightly related questions:

1.  Why did XML keep the requirement for deterministic models?

2.  What are good reasons for having multiple schemas for the same 
    instance?

The first is related to a wish list from the X3D spec group for 
schema revisions.  The second is a best practices question for 
those who use schemas as normative text in standards and specifications. 

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