Hilarious, but the deeper rule is: when there isn't a concept of rendering a document to the user (where attributes hold rendering properties not directly prese

Hilarious, but the deeper rule is: when there isn't a concept of rendering a document to the user (where attributes hold rendering properties not directly presented to a user) the concept of a markup language doesn't make sense in the first place. So if you find yourself in a situation where you need to decide on encoding data as elements vs attributes, you probably shouldn't use SGML/XML at all. But then there is the wealth of XML standard data representations for financial/fiscal/e-commerce, medical, etc. data. You're deciding to use them to have your interfaces being implemented once and for all, and be able to switch eg. payment providers by merely specifying new endpoints and credentials. JSON doesn't really compare to XML/XSD in this space.