Interesting article from daisy.org about fallbacks being disused:
Quote:
Manifest fallbacks are named for how they are implemented in the EPUB package document. The manifest element is used to list all the resources used in the publication, and each item in it can specify another item as a fallback using the fallback attribute.
There are two places that manifest fallbacks are used. The first is to include unknown media types in the spine (i.e., to make them documents in the publication reading order). This practice has largely fallen into disuse, however, as it was used primarily in EPUB 2 to allow SVG images in the spine with XHTML fallbacks.
Code:
SVG in spine (EPUB 2)
<manifest>
<item id="c01" src="images/plate01.svg" media-type="image/svg" fallack="c01-xhtml"/>
<item id="c01-xhtml" src="images/plate01.xhtml" media-type="application/xhtml+xml"/>
…
</manifest>
<spine>
<itemref idref="c01"/>
…
</spine>
In this case, if the reading system does not support SVG, it follows the fallback attribute references until it finds a media type it can support. In the above case there is only one fallback, but it is possible to keep falling back through multiple items before reaching a supported media type. The only requirement on fallback chains is that they must include a core media type.
The other place where manifest fallbacks are used is to provide a fallback image for the HTML img element. Until the recent addition of the srcset attribute and creation of the new picture element, it was not possible in HTML to provide a fallback for images embedded using the img element. As a workaround, reading systems are expected to change the source of the img element to the fallback specified in the manifest.
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