As a programmer, I really liked XHTML because it meant I could use a regular XML parser/writer to work with it. Such components can be made small and efficient if you don't need the more advanced features of XML (ex: schemas), on the level of JSON. I remember an app I wrote that had a "print" feature that worked by generating an HTML document. We made it XHTML, and used the XML library we already used elsewhere to generate the document. Much more reliable than concatenating strings (hello injections!) and no need for an additional dependency.

In addition, we used XSLT quite a bit too. It is nice being able to open your XML data files in a web browser and having it nicely formatted without any external software. All you needed was a link to the style sheet.