The digital book contains backwards compatible equivalents (EPUB2) where necessary.
Many users still have older reading devices and software that do not support EPUB 3. By providing backwards compatibility with EPUB 2, publishers can ensure their ebooks are accessible to a wider audience.
Objectives
Ensuring playback on older devices
Implementation
Use EPUB2 cover metadata
Include a table of contents in toc.ncx format
Add EPUB2 guides to convert EPUB3 landmarks
Provide fallbacks for HTML5 interactions
apply a reset to the HTML5 elements used so that they do not pose a problem for solutions that do not support them
Place media queries in a separate CSS sheet
Control
Check the presence in the ‘opf’ file of the ‘cover’ metadata
Check for the presence of a toc.ncx file
Check the presence of a section in the opf file
Check for the presence of alternative content for HTML5 elements
Check the presence of css information for HTML5 elements (article, aside, details, figure, figcaption, footer, header, nav, section)
Check CSS sheet separation if layout is done using media queries