The included objects are provided with an appropriate fallback alternative.
Some digital books contain particular objects such as interactive javascript elements (form, map, etc. ) that may not be available in certain contexts. An alternative allows you to access their content.
Objectives
Provide information access for users whose reading device does not support the inclusion of objects or technologies used in included objects.
Facilitate the exploitation of these contents by robots.
Improve accessibility of content to readers with disabilities.
Improve content support by search engines and indexing tools
Implementation
Use intrinsic fallback mechanisms (such as those available for object [html] and canvas) or, when an intrinsic fallback is not applicable, using a fallback at manifest level. Fall chains are created using the Fallback attribute on the elements of the manifest. This attribute refers to the xml ID of another element of the manifest which is a fallback solution (Fallback) for the current element.
Control
Make sure the digital book remains readable and usable on an old device or by disabling javascript support
Make sure content of type object or canvas is not necessary for understanding or have a textual alternative.