I think it's more appropriate to say XML is about a grammar description of some information serialization. Whereas JSON starts from concepts such as maps which

I think it's more appropriate to say XML is about a grammar description of some information serialization. Whereas JSON starts from concepts such as maps which are higher-level data structures. Also, SGML/XML originally isn't so much about trees as about regular tree languages - where the sequence of elements appearing below a node must satisfy a content model given by a regular expression.