PDFs don't use HTML. Standard PDFs are also not XML based. PDF.js ab-uses HTML to somehow render PDF. The HTML used by PDF.js is in no way a sane "human read

PDFs don't use HTML. Standard PDFs are also not XML based. PDF.js ab-uses HTML to somehow render PDF. The HTML used by PDF.js is in no way a sane "human readable structured data" format. But a "browser compatible" ad-hoc representation for the underlying PDF representation. Lastly PDF are not formats for structured data, which is what XML is all about in the end. So I have no idea why you thing HTML+Js is a sane replacement for XML use cases, it isn't. It isn't a replacement to a degree that IMHO it doesn't even really makes sense (the JS part).