Each HTML identifier is only used once per Content Document.

On the page of a book, we can identify certain elements by an identifier. It must be unique within this page.

Objectives

• Avoid display problems
• Guarantee access to the identified element

Implementation

• Assign unique identifiers to each element in the code of HTML pages ensuring that no identifiers are reused within that page.

Control

• Check the source code of the epub HTML page: No HTML identifier must be used more than once. Or EpubCheck

Validation

• Needs to be human verified.