In the XML world, XML documents can live their own lives independently of programs: they can be edited, read, displayed, and transformed using generic tools independent of any particular application. It's also vitally important that they can be validated independently of any application. This validation requirement presents a serious challenge. The diversity of XML vocabularies is virtually infinite. We certainly don't want to limit XML's extensibility because of the tools used to validate XML documents. But that brings us to the next problem: there is diversity in what we can call validation.

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This application independence raises some difficult issues in XML design and usage. Some people have focused on the surface parallels between XML document structures and object hierarchies. They say that XML is in the same paradigm for data as object orientation and that XML is a perfect serialization format for object systems. While that assessment is not completely without basis, XML reintroduces a clean separation between data and processing. This is the complete opposite of the basic object-oriented principle of encapsulating both data and behavior into objects.