In content implemented using markup languages, elements have complete start and end tags, elements are nested according to their specifications, elements do not contain duplicate attributes, and any IDs are unique, except where the specifications allow these features.
The purpose of this success criterion is to ensure that critical markup errors do not cause user agents and assistive technologies to interpret documents differently.
How to Meet
EPUB Creators should use a validator, such as epubcheck, to validate their content. Although not all the issues flagged by a validator must be fixed to meet this success criterion, they will normally flag the major issues that result in failures.
Note that this requirement is closer to ensuring all XML content is well-formed than that it fully validates to its respective standard.
The following knowledge base pages provide more information about how to address this success criterion for publishing content: