I don't think there's anything wrong with using HTML/XML-ish format for describing a UI (although having a standardized presentation format that all "viewers/browsers" follow exactly the same way would be nice), I'm just sad that websites have become described as UIs rather than as well-structured documents.

PDF is a way to make sure everyone can read the document in the same way (like font type, spaces, special characters...) while HTML can cause multiple issues if you don't do it properly. And for documentation I would prefer some Markdown as I can edit it with a fancy IDE or feel lazy and do it with terminal editor or plain text editor.