XML API - robots exchange XML, converted to human form with a stylesheet, both on the fly and stored documents. From the description of fields to full blown app

XML API - robots exchange XML, converted to human form with a stylesheet, both on the fly and stored documents. From the description of fields to full blown application, think XSLX plugged Excel stylesheet. That's how desktop applications work, maybe we should ask why is it not the norm? Why each request downloads application intertwined with data? Why can't one save response as one saves desktop document? I am ok with JSON but I want a way to plug application supported by browser, XML is the only choice yet.