A use case becoming more relevant is audio. Epub2 does not support audio and audio data in custom fields is non-portable. Epub3 provides portable audio support AND has mechanisms to syncronize playback with highlighted text a la Kindle. Amazon is now pushing audio books for readers on the go and many cars now allow audio to be played though their speakers via bluetooth. I much prefer to listen to books while driving rather than listening to the garbage on the radio. Please reconsider support for Epub3!
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