XHTML may not be mainstream with people authoring web pages, but it is a thing within publishing pipelines that generate XHTML from other XML-based sources like

XHTML may not be mainstream with people authoring web pages, but it is a thing within publishing pipelines that generate XHTML from other XML-based sources like JATS or DocBook. It's also a thing at least in ePub 1 and 2 content (I can't recall if they relaxed it to HTML for ePub 3 or later).