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

Validation

• Needs to be human verified.