What is essential for EPUB media overlays?

The essential aspect of EPUB media overlays is the provision of an accessible playback experience that ensures synchronization between text and audio. This feature allows users to follow along with the text as it is read aloud, which is particularly beneficial for individuals with visual impairments or reading disabilities. By synchronizing audio with written content, media overlays enhance comprehension and engagement for users, making digital publications more inclusive and user-friendly.

The other options, while they may contribute to overall functionality or appeal of digital content, do not address the core utility of EPUB media overlays. For instance, the ability to add images at any point or automatic translation into multiple languages are valuable features but do not specifically relate to the synchronous experience that is vital for accessibility in media overlays. Similarly, integration with third-party applications could enhance user experience, but it is not a defining characteristic of what makes media overlays essential.