If you’ve seen this message from epubcheck, you know where this post is going.
It’s easy to get tripped up on EPUB’s web-like, but not quite web, quirks, as what is valid to do on in a web page isn’t always valid to do in an EPUB. Particularly when it comes to linking to resources, as you have to follow EPUB’s core media type requirements.
If epubcheck has spewed the “hyperlink to non-standard resource” message at you, it’s because you can only have internal links go to XHTML or SVG documents, at least without a fallback.
But don’t fret. There are some easy, and not-so-easy, workarounds to this problem, which is what I’m going to look at.