Thanks for the history lesson.
My point about strings and children still stand though. SGML is a document markup language. It was never designed for complex scene graph work, so I don't get how the OP I responded to came to the conclusion that HTML derived from anything that was originally used for things like applications and GUIs.
No, its not.
HTML is based on SGML (it was originally an SGML application profile, though as of HTML5 it is no longer.)
XML was invented after HMTL, was inspired by HTML, and is also an SGML application profile.
So, insofar as XML and HTML are related all, its closer to the reverse of the relationship you describe.