I have noticed that if you write an <a href="data:application/xhtml+xml;..."> link inside an EPUB, it will pass epubcheck but it will break quite a lot of readers. In the case of Apple Books, you will not be able to navigate back unless you trigger a relayout of the content.

I am trying to understand if this is a bug in the readers, a behaviour out of spec, or just a grey area. I gather that data URLs used in this way are implicitly disallowed by the spec, because you can't point to them in the manifest or in the table of contents. Can somebody please clarify? If this is non-compliant behaviour, then it should be checked by epubcheck, I suppose.

For context, LaTeXML produces EPUBs snippets of code embedded into data URLs, causing the problems I just mentioned.

Minimal example attached.