It's not like HTML except for being XML which looks a lot like SGML.
It's significantly more limited and more specific to the task than HTML. HTML comes with a lot of baggage that makes it unsuitable for a really specific use case.
It's not like HTML except for being XML which looks a lot like SGML.
XML does not only look a lot like SGML, it is a proper subset of SGML:
XML is an application profile or restricted form of SGML, the Standard Generalized Markup Language [ISO 8879]. By construction, XML documents are conforming SGML documents.
It's significantly more limited and more specific to the task than HTML. HTML comes with a lot of baggage that makes it unsuitable for a really specific use case.