Sigil does not work well with ePub 3. You load in an ePub 3 ePub and it will become corrupted.
ePub 3 is a poor choice for a source format unless you are using features in ePub 3 that are not possible to use in ePub 2. But most eBooks these days that are being turned out as ePub 3 have np need at all to be ePub 3 and would work perfectly as ePub 2.
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