[KevinH]

Jon, please stop attacking everything that is epub3. CSS3 is valid in epub3 and rem is different than em in epubs that use different font sizes in different places as it always references the size of the font in the root (html) element.

Epub2 is now totally dead. Deprecated almost 15 years ago (first approved epub3 spec was dated, October 11, 2011). Nothing new should ever be created in epub2.

Good, spec compliant Epub3 readers are now broadly available and making epub3 backwards compatible with the old epub2 spec is possible.

This forum's love and support for a long dead specification is really misguided.

Fair warning: At some point, Sigil will eventually drop support for creating new epub2s and when asked to load one, it will eventually use integrated code (based in my ePub3-itizer plugin) to generate an epub3-only version for editing and saving.