Although EPUB 3 is not perfectly backwards compatible with EPUB 2, a goal of the recent revision was to ensure, as much as possible, a basic level of rendering when features overlap.
In this thread, we’d like to compile a list of best practices that content creators should consider implementing for optimal rendering in both EPUB 2 and 3 reading systems, as many readers are likely to be consuming the new format initially on older systems. (As these reading systems get updated to support the new standard, this information will naturally become less relevant to production.)
In that light, the following practices are recommended at this time:
- An NCX should be included to ensure navigation and so that reading systems don’t generate errors due to its absence.
- Any styling employing CSS3 properties needs to degrade gracefully, as CSS3 support was not included in EPUB 2.
- Progressive enhancement techniques must be used for scripting as per the EPUB 3 specification, as scripting will not be available on many EPUB 2 reading systems.
- The epub:switch element should be used to include fallbacks for embedded MathML content.
- Embedded fonts may not render, so optimal fallback fonts should be listed in your CSS style sheets.
- Audio and video content are not likely to render using the new HTML5 elements, so embedding the object element as a fallback is a possibility for systems that supported multimedia (providing transcripts should also be considered, as many will not)
Please post any pertinent information and questions in this thread and we will update the above list accordingly.