You can use them for EPUB 2 compliance (SVG with
XHTML fallbacks, as SVG wasn't allowed in the spine previous), and even for script-less reading systems (fallback from an EPUB 3 - compliant XHTML content document marked as scripted to another EPUB 3 - compliant XHTML content document not marked as scripted).
Not exact matches
However, it is possible to include a reference to a MathML file in the ePub book, perhaps as an «examples» page, with a
fallback to an
XHTML equivalent (perhaps one which includes static image examples).
Here I defined a document with an unsupported media - type (text / html) and a
fallback in a supported type (application /
xhtml + xml).
Publication Resources referenced from
XHTML and SVG Content Documents and CSS must now be Core Media Types unless referenced in a context that provides native intrinsic
fallback capabilities.
The default element acts as a
fallback for the switch and must include a representation of the content that is valid in
XHTML Content Documents.
You'll have to add a
fallback attribute that points to the entry of either an
XHTML or SVG
fallback:
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.
What bindings do is allow you to attach a scripted
XHTML document to another media type as a first
fallback.
And once again, since only
XHTML and SVG content documents are allowed in the spine without
fallbacks, you either have to wrap the resource up as
XHTML or SVG or provide a
fallback.
An intrinsic
fallback is one available by default in the host grammar, whether that's
XHTML, MathML or SVG.
Since the
fallback is going to be an
XHTML or SVG document containing the resource, there's almost no point in bothering to reference core media types directly.
These alternate Rendition
fallbacks might be used, for example, so that a formatted graphic novel defined via a sequence of SVG pages can be accompanied by an accessible text version defined via
XHTML.