[em-bee]

Those were defined as machine language meant for machine talking to machine

i don't believe this is true. machine language doesn't need the kind of verbosity that xml provides. sgml/html/xml were designed to allow humans to produce machine readable data. so they were meant for humans to talk to machines and vice versa.

[soulofmischief]

Yes, I think the main difference is having imperative vs declarative computation. With declarative computation, the performance of your code is dependent on the performance and expressiveness of the declarative layer, such as XML/XSLT. XSLT lacks the expressiveness to get around its own performance limitations.