先のエントリで紹介した米国の出版団体 Association of American Publishers (AAP) のEPUB 3 Implementation Project 報告書には、 EPUB 3.0 の36の機能の中を調査し、機能の重要度、そして、実装の難易度の観点から、EPUB 3 リーディングシステムに優先的に実装してもらいたい10の機能が順位をつけて掲載されています。

• EPUB 3 Implementation Project White Paper Now Online | The Association of American Publishers
• AAP EPUB 3 Implementation Project | The Association of American Publishers

　AAPの報告書という形でEPUB 3の機能の実装について優先順位を公に示し、共通認識とすることで、リーディングシステムデベロッパーの参考に資することを目的としているようです。
　報告書の該当部分を以下に転載します。

Specific EPUB 3 Features Ranked as High Priorities

1.Navigation

• TOC: A linked table of contents is a high priority. The EPUB <nav> uses HTML5 markup that is accessible and easy to create,
  replacing a cumbersome navigation format in EPUB 2 (NCX). Elimination of dependence on the NCX by reading systems and its use by publishers is a high
  priority.
• Page-list: An optional mapping to print pages as part of navigation. Offering page-mapping enables users of print and ebook to be “on
  the same page” whether in a book club or the classroom, and enables textual references to page numbers (e.g., indexes, cross references, and citations) to
  work. It is considered critical for accessibility.
• Landmarks: An optional short table of contents, the landmarks nav can be used to access key elements of book. It is easy to implement.

2. Audio:The EPUB spec requires support for one of two audio formats (this is a prerequisite for media overlays):

• MP3, currently the more popular and common audio format
• AAC, the audio format within the MP4 multimedia format

3. SVG:Scalable Vector Graphics are XML-based vector images that can be zoomed infinitely without loss of clarity. This is important to publishers, which can use
SVG images without grappling with responsive design. Further, SVG is accessible in that the content is machine readable and can work with Braille readers
and even 3D printers.

4. Fonts:It is important to see support for fonts and information about fonts in EPUBs and EPUB 3 reading systems. Fonts are not just for design purposes; they are
often needed in order to render characters that are not part of default character sets. This is common in non-English words and for special characters,
such as those used in phonetics, physics, or math. Important font issues are:

• Font descriptors: The characterization of font data. Support for basic CSS font descriptors without relying on work-arounds is a high
  priority:
  • font-family (at least serif and sans-serif, without font embedding)
  • font-weight
  • font-style
  • font-size
  • font-face
• Embedding: Inclusion of font files inside the EPUB package.
• Font obfuscation: A standard method for inclusion of font files inside the EPUB package such that they cannot easily be exported for
  usage outside the publication. (Note that this is not the same as encryption, which is required by some font licenses.)

5. Media Overlays: Representation of audio synchronized with text content. This can be a visual display, such as a bouncing ball highlighting words in an early reading book,
or it can be an audio book/ebook that keeps one’s place between reading and listening. Use of Media Overlays is particularly important for publishers in
segments such as children’s books as well as for the accessibility community,

6. Semantic inflection (e.g., page-break, part, chapter, index, glossary, sidebar, footnote): an attribute that allows for precise statements to be made about markup within HTML
(e.g., “noteref” and “footnote”). This allows for precise and uniform markup and rendering across publications and reading systems, allowing for contextual
search and improved navigation.

7. MathML: An XML vocabulary for describing mathematical notations and capturing both its structure and content. MathML allows for reflowable, resizable, searchable
math equations. It is considered crucial for STEM and Education publishers, otherwise math must be displayed as static images. It is also critical for the
accessibility community, as images are meaningless and descriptions of images do not convey the same meaning as the MathML.

8. Video:The EPUB specification offers two options for video support. Neither is a requirement. However, if video is not supported, the reading system should
support a fallback to an image. Being able to “not support” video properly is as important as supporting video, so that each publisher can create one file
for all retailers. The two video formats supported by EPUB 3 are:

• H.264, currently the more popular video format
• VP8, a royalty-free format increasingly used on the web

9. Float: The CSS property that enables elements to be positioned independently of the linear narrative flow. This is often a property of the underlying system or
browser upon which the reading system is built.

10. Fixed Layout (FXL): A method for creating static, fixed e-book “pages“. Some content is better suited for a “fixed“ instead of reflowable display. This is important to some
segments of the publishing industry.

from “II. Priorities for EPUB 3 Reading System Features” AAP EPUB 3 Implementation Project 報告書より

　
　同報告書では、コンテンツ作成者向けのアクセシブルEPUB 作成TIPSも掲載されていますが、そののTIPSと以下のリーディングシステムデベロッパー向けの10の優先順位は必ずしも対になるものではないようです（以下の10の機能を実装するだけでは、TIPSに従って作成したアクセシブルなEPUBの機能を全て生かせるわけではない）。

関連エントリ

• 米国の出版団体AAPのEPUB 3 Implementation Project 報告書
• EPUB を アクセシブルにする13のTIPS（AAP EPUB 3 Implementation Project 報告書より）
• Accessible EPUB Reading System – DAISYユーザーのためにEPUB 3リーディングシステムが求められること