Now that we reference the WHATWG HTML spec, custom elements will become valid in EPUB 3.3, which are kind of the nightmare scenario for epubcheck since they aren't evaluated until documents load.

But putting aside the general inability to enforce rules on them, @rdeltour asked in w3c/epubcheck#1288 about their impact on the remote resource rules.

While I believe the underlying requirements remain valid, it's not easy to check how the resources will be used not knowing what the custom element extends or translates to from epubcheck.

So the possible options are:

1. Scrap the explicit restrictions and rely on media type checking and whatever heuristics epubcheck has for determining what are legitimate remote resources
2. Add another bullet that allows any of the four types of exempted resources to be referenced from custom elements

I tend to lean toward the first option for pragmatic reasons -- it's probably what the second is going to force epubcheck to do anyway -- but if we want to keep the current specificity we could go with the second option.