A word document is xml
Lots or file formats are just zipped XML.
I was reverse engineering fucking around with the LBX file format for our Brother label printer’s software at work, because I wanted to generate labels programmatically, and they’re zipped XML too. Terrible format, LBX, really annoying to work with. The parser in Brother P-Touch Editor is really picky too. A string is 1 character longer or shorter than the length you defined in an attribute earlier in the XML? “I’ve never seen this file format in my life,” says P-Touch Editor.
Sounds like it’s actually using XSLT or some kind of content validation. Which to be honest sounds like a good practice.