[skreutzer]

Replacing <u> with a CSS class won't break a book. <u> might break a book or might have no effect or might have a different effect than underlining or might underline. Why should I speak to Apple, Kobo and B&N? I don't use Calibre, my EPUBs are valid. At least Apple was involved in the EPUB3 specification process, so they probably won't violate the standard they've helped to develop in order to create a common protocol for e-book interchange and consumption.