Clarifying Baseline Read-Aloud Expectations for Media Overlays and Text-to-Speech in EPUB #2860
Description
Describe the problem
EPUB supports read-aloud experiences through multiple mechanisms, most notably Media Overlays (synchronized, publisher-provided, pre-recorded narration) and Text-to-Speech (TTS), in which narration is generated synthetically by reading systems.
Although both mechanisms are referenced across EPUB specifications, their conceptual roles, guarantees, and limitations are not clearly articulated in one place. This has led to:
- confusion among authors and publishers about what EPUB actually guarantees (or does not guarantee) for read-aloud behavior;
- conflation of synchronized narration and synthetic speech as interchangeable solutions; and
- repeated attempts to standardize technologies or behaviors that lack consistent implementation support.
The absence of clear baseline guidance discourages adoption of read-aloud features and leads to unrealistic expectations regarding consistency, granularity, and behavior across reading systems.
This issue concerns editorial clarification of existing EPUB concepts and behavior only. It does not relate to specific reading systems, styling questions, validation errors, or the introduction of new features.
Describe the fix or new feature you propose
This issue proposes editorial clarification to existing EPUB specifications in order to:
- explicitly distinguish synchronized narration (Media Overlays) from synthetic narration (Text-to-Speech) as separate read-aloud mechanisms with different guarantees and use cases;
- document realistic baseline expectations for read-aloud behavior in EPUB, including expected variability across reading systems; and
- improve cross-references and guidance across existing specification sections where Media Overlays and Text-to-Speech are discussed.
No new formats, APIs, media models, or conformance requirements are proposed.
The intent is to clarify existing concepts and align documentation with current practice, not to redefine or extend EPUB’s read-aloud capabilities.
A detailed proposal describing the motivation, scope, and intended editorial changes is attached.
Clarifying Baseline Read-Aloud Expectations in EPUB, v2.2.pdf