API to An Aggregate Document

A theoretical question.  Given an XML aggregate 
document such as HTML inside SVG inside X3D, 
what should one expect the API to do?  XML 
DOM is generic.  HTML DOM and SVG DOM add 
their own API extensions.  An author learns 
one or all in their separate browser objects. 
Then one sunny day, they use namespaces and 
aggregate these.  Do they get to leverage 
what they have learned?  How?

If the promise of XML is integration, how 
far does that go in the scripts?

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