Adding CSS support to FBReader is a major upgrade, but it is apparently being worked on.
The ePub TOC support is easier, but does seem to be a road block (since MobiPocket have not done it either). It is in fact possible to add a 2nd TOC in the .opf file, and this one is honoured by FBReader and MobiPocket. Many older ebooks have a TOC inside the ebook itself near the start of the book, and these work as usual but are not associated with a TOC icon.
DRM-free ePub probably is the best format they have in common. However, the use of Calibre to format shift for the PRS-505 is very common. For DRMed content this first requires breaking the DRM, which is possible with MOBI and LIT and eReader formats. All of these convert well using Calibre, either to LRF or to ePub. The ePub is readable on the iLiad, but it is also possible to start from the original and format shift differently for the iLiad. The most recommended initial format is LIT (MS Reader). This can be exploded into an OEB ebook for FBReader on the iLiad, and converts well to ePub as well. One advantage of LIT is that it tends to have larger images than the alternatives, and the large images are better for the iLiad.
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