The current text on XML conformance says that an XML-based media MUST meet:

...
External identifiers MUST NOT appear in the document type declaration [XML]
...

This constraint apply regardless of whether the given Publication Resource is a Core Media Type Resource or a Foreign Resource.

However, that creates problems with SVG 1.1. The conformance rules of SVG1.1 require the presence of:

<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" SYSTEM "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">

The DTD itself, as described in the relevant appendix, is making a heavy use of external entities (via its usage of DTD Modularization). As a consequence, per the current document, no conform SVG 1.1 file is valid for EPUB 3.3.

Although the upcoming SVG 2 removed this requirement, there are lots of perfectly valid SVG 1.1 out there (SVG 2 is not yet a Recommendation, b.t.w., only a CR). Most notably, Adobe Illustrator generates SVG 1.1 files with those DTD-s in them.

A similar problem may occur with MathML 3, whose official DTD relies on the same approach (although MathML is more permissive and it does not require the usage of that DTD).

This problem was originally raised in w3c/epubcheck#1114.