Simultaneous reading, A11Y, and alt="" (re: bitmaps as spine items) https://groups.google.com/d/msg/epub-working-group/UCAa9l5IOKk/tbXsoyBzcPMJ 4月にEPUBのアクセシビリティに関して、schema-orgの案（accessMode）を検討する価値はあるよと。Images-in-spineはcf https://groups.google.com/forum/#!topic/epub-working-group/4A13khT1j8o