Declarative Capability Intent Metadata for EPUB Publications #85
Description
Problem Statement
EPUB publications often rely on optional capabilities such as media overlays, scripting, interactivity, or fixed-layout behavior. Today, these assumptions are implicit: authors encode content expecting certain capabilities to be available, while reading systems silently ignore unsupported features or apply unspecified fallbacks.
This implicit model creates several problems:
- Publications may degrade unpredictably when expected capabilities are unavailable.
- Authors and tool vendors rely on undocumented conventions and trial-and-error testing.
- Accessibility outcomes vary depending on reading system behavior.
- Libraries, educators, and platforms cannot reliably assess suitability or behavior before distribution.
- Discovery and filtering of content based on capability expectations (e.g. read-aloud support or non-scripted content) is not possible in a standardized way.
Although many of these expectations are understood informally by experienced implementers, they are not currently expressible in a standardized, inspectable form.
Proposed Fix / Solution
Introduce Declarative Capability Intent Metadata: a lightweight, optional, and non-enforcing mechanism that allows an EPUB publication to declare the capabilities it:
- relies on,
- prefers,
- tolerates, or
- assumes absent.
This metadata expresses authorial intent and assumptions, not requirements. It does not mandate reading system behavior, disable features, or affect conformance. Reading systems and platforms may ignore it entirely or use it for informational purposes.
By making implicit expectations explicit, this approach:
- reduces silent failure and unpredictable degradation,
- supports accessibility auditing and discovery,
- enables libraries and platforms to search and filter content based on capability expectations (e.g. read-aloud support, non-scripted content),
- improves transparency and trust across the EPUB ecosystem,
- and remains fully backward-compatible with existing EPUB publications and reading systems.
Many of the expectations addressed here already exist informally in today’s EPUB ecosystem; this proposal makes those expectations explicit, inspectable, and shareable.
v 0.2, Proposal_ Declarative Capability Declarations for EPUB Publications.pdf