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My understanding is that XML was derived from document-oriented SGML, to beat SGML into a form that would work well with XSL and XPath.

But I'd like to point out that the way SGML-derived markup distinguishes attributes and child nodes is entirely arbitrary. You could as easily make attributes child nodes - it's all in how you interpret what's written. Likewise, you can "convert" SGML to JSON (or YAML or S-expr or whatever) very easily, bearing in mind that attributes and child nodes sit in the same space with each other - a well-formed XHTML document, for example, can be re-expressed in JSON without ambiguity, since tags have a well-specified, unambiguous list of allowed children and attributes - just give text nodes the name "text" and you're golden.