I saw a quick flash of unstyled content (in Firefox) where all the text was jammed into one paragraph with no breaks. And then the page rendered normally. So I glanced at the source code, and it surprised me to see that it was not a typical HTML document, but instead an XML document with a stylesheet. It is very rare to see a non-HTML page on the web, whether it is the XHTML serialization of HTML, or fully custom XML…
I agree this is very unusual. It's also somewhat annoying, for example I can't just send this to my Kindle to read it, and I can imagine other tools failing on this as well.
Seems DocBook has been around since 1999, judging from when the first O'Reilly book was published about it [2].
I just flagged it with the Pocket app's "Report An Article Display Problem" feature, but it is such a niche use case that I can't imagine them spending time on it.
[1] https://en.wikipedia.org/wiki/DocBook
[2] https://www.oreilly.com/openbook/docbook/book/docbook.html