I have a feeling that industry is moving away from XML and towards JSON...
Just a feeling, but even HMRC, who used to used XML for VAT and do for RTI, have moved to JSON for MTD (VAT).
Fundamentally XML is too complicated - especially with name spaces. The options in XSD (which is the common way to define what is valid in an XML file) is also a bit of a mess - defining the order of the XML objects even, which is a pain if you ever work with any XML. JSON does not expect values in an object to be in any order (just in an array).
As it happens, at A&A, we have been working on new APIs for our control systems (more on that another day), and these are all JSON based too. But this week I have started on some work on the accounts system.
I now have JSON for documents, i.e. invoices. We currently allow customers to access invoices in XML, PDF, Plain text, and HTML, and as of now also JSON. You can ask for a JSON attachment to be automatically included in your invoice emails if you wish - digitally signed, obviously.
Whilst we have loads of work behind the scenes improving the way the accounts work, this JSON interface should be pretty stable now.